He was clever. If he invested that time and money and she still wanted out, her salary would be much closer to his, which would have worked out in his favor when it came time to calculate child support or alimony. It's possible (maybe unlikely) that if she had made more than him that _he_ could have ended up receiving money in the divorce.

In fact, I can't see a down side in this case for either (adult) party -- being poor and uneducated with child support is inferior to being relatively self-wealthy with modest child support, so it was a win for her.

There's a value to raising your children yourself; a stay-at-home parent can focus on the needs of the children and spend more time on giving them a first class childhood experience. If the relationship is headed for the wall, however, it surely is superior to do this at the expense of trying to continue stay-at-home parenting.