There's actually studies on this subject, a woman out earning her husband is more likely to leave the relationship. The OPs friend got a bad deal, sucks when it happens. Not only did do he lose time that he can't get back, but money he can't get back.
You just have to notice the red flags, I had a relationship that ended not too long ago, and we had talked about buying a house together, she told me that she didn't want to pay half the mortgage because it would be like we were roommates, but she still wanted her name on the mortgage lol.