I don't really see the problem here. If a man is into a woman, it's not unusual that he'd be into her daughter/mother. They're going to share genetics so likely be of similar beauty caliber and personality. Kind of like all the stories of guys sleeping with the sister of their gf/ex or how women that was dating 1 guy in a friend's group ends up sleeping with all of the guys in the friend-circle because generally they're all somewhat similar.

Now that I'm approaching my late 30s, it's not unusual at all for some single mother to want to date me and me thinking to myself I'd rather just date her daughter.

The only thing I find odd about this story is that it's usually the other way around. Where the daughter steals the mom's bf. Rare for it to go the other way around. I bet everyone is happy about it though. The mother is probably better suited for him than the daughter and the daughter was just living out some daddy issues with the guy.