The initially secular husband found himself attracted to the kiruv he encountered. He then delved into frumkeit via a very knowledgeable & caring rav.
His secular Jewish girlfriend at the beginning of his kiruv refused to follow him on this journey, so they broke up and he married someone else who loved him deeply—but with whom he never felt fully satisfied.
She was pretty with good middot & a nurturing personality. She came from a wealthy home with a tennis court & a swimming pool, and she only ever wanted to become a little bit more frum than she'd been raised, but she willingly followed him into ultra-Orthodoxy & a kollel life, which meant living in deprivation.
But though she embraced it all with a good attitude, he never managed to really appreciate her.
Over time, he forced his growing family into a very insulated frum community, including speaking Yiddish at home (which his wife never managed to learn well) and sending them to chassidic Yiddish schools (carefully avoiding telling the school that they weren't FFB—which is never a good sign; if you have to fool the school, you probably don't belong there).
Observing the appalling way he often spoke to & treated her felt very discomfiting. He seemed to resent her for no reason.
But except for the occasional times she reached her limit in patience, she usually accepted his poor behavior with equanimity & even an understanding smile—as was her nature.
Then the rav passed away & things started to really deteriorate.
The problem was that the husband's connection with the rav acted as the glue that held the husband to frumkeit.
Sure, all the learning also felt good; wonderful ideas exist in our holy books. But the main attraction for him was the rav.
In addition to growing up in a cold society, the husband always lacked a father figure in life too, and this wonderful rav provided warmth, love, and direction.
When the rav passed away, that source of warmth, love, and direction disappeared too.
The husband's treatment of his wife & children worsened and his grip of Yiddishkeit also deteriorated.
Via social media, he found the ex-girlfriend (who was still secular and now married to an equally secular Jew) and he ended up spending tons of money (further depriving his own family) to conduct an affair until her husband put a stop to it by threatening to divorce her & deny her all custody of their children.
Gradually, all the formerly frum husband's children went off the derech while his wife took sedatives to deal with the situation.
Chazal speaks about this, that a man's debauchery will deprive his wife & children of their rightful parnassa, and influences them to behave badly too, whether they're consciously aware of his transgressions or not.
The couple divorced and he left the children in her custody with barely any financial resources, not paying child support & only the most occasional visitation.
The wife (or ex-wife by now) mostly drifted out of frumkeit, except she maintained her Shabbat observance & struggled for several years to get on her feet financially. (Actually, I don't know if she ever did, or if she continued to live off of sympathetic family & government programs).
He took his cushiony new livelihood and went on to marry a financially independent woman and start a new family.
And up to here is all I know.
2 Lessons: Derech Eretz Kadmah l'Torah & The Erev Rav Psyche
They belonged to Moshe Rabbeinu. The Erev Rav are called his—Moshe Rabbeinu's. In speaking to Moshe Rabbeinu, Hashem calls them "YOUR people that YOU brought up from the land of Egypt (Shemot 32:7)."
He fought for them (Hashem hadn't wanted them). They followed a leader whom they expected to take care of them.
They never fully connected to Hashem & Torah.
Likewise, in the above example, the man looked like a sincere baal teshuvah on the outside, but his real connection to frumkeit was via this special rav.
Any kiruv, any rav or rebbetzin, any shiur is only ever a MEANS to connect to Hashem & Torah.
It's a way to learn.
Most people use them that way. It's obvious.
But some people don't.
And that's a defect in them.
Secondly, there's also the idea of Derech eretz kadmah l'Torah—that common decency precedes Torah.
Meaning that if you're a total jerk, you're not going to be able to follow Torah properly.
You have to WANT for Torah to influence you positively.
But if you're a jerk, then every time you encounter a situation in which the upper road feels hard & awkward, you simply take the lower road.
That's why the man in the above situation treated his wife & children so badly. Most of the times he needed to hold his tongue, speak nicely, or be considerate, he simply refused and behaved however he felt.
(BTW, people who do this generally feel like they're the victim, which allows them—in their own mind—to abuse others. They honestly think they don't NEED to be nice because the perceive the other person as the one victimizing them!)
His self-indulgence of bad middot led to snarly bouts of depression (being a jerk is pretty depressing) and eventually slid into the terrible sins of disloyalty to his own wife & aishet ish with somebody else's wife, until the final descent into complete secularity.
Because of his lack of derech eretz & his desire to fulfill his emotional needs (rather than his soul needs), he never managed to embrace the essence of Judaism.
So despite his extremely frum-looking external appearance (wearing chassidish clothes) and his external behavior (living a kollel lifestyle & speaking Yiddish), he was always a fake (even though during his phase with the lovely rav, he FELT sincere).
It's heartbreaking that his wife & children got both duped & dumped in the process.
But that's what happens when derech eretz doesn't precede Torah and also when a person pursues Judaism for ego fulfillment without ever transitioning to soul fulfillment.
Just like the Erev Rav.
We also see from the above why the Erev Rav pack such a devastating punch against the mitzvah-observance of Am Yisrael. They suck all the meaning right out of everything, leaving you with a cloudy, painfully false experience of Torah.
Despite the grueling challenge they present, it's up to us to look behind the smoke-and-mirrors of the Erev Rav. They believe in what they're doing, which makes it harder for us to see behind their façade.
Reading the last part (the section entitled "Melave Malka") of the above-mentioned post sheds the necessary light on why they are so hard to perceive.
But that's a big part of our avodah: seeing the Truth.
(Note: It's routine for baalei teshuvah & converts to enter Torah observance with a combination of motives. That's normal & to be expected. But at some point, inner work needs to occur & at least the beginning of a transition needs to be made into internalizing the Torah's essence & values. For converts, the conversion process really needs to do this, the responsibility of which lies on the rabbi involved. But either way, the transition needs to happen at some point for all Jews.)