I think the point OP is making isn't "it's strange that the mother had such a boyfriend", but rather "it's strange that so much forensic effort was required to suspect the boyfriend when he should have been an obvious first guess".

I mean, there is no official database of “boyfriends”. Even after you identified the girl, and found the mom, how would you figure out if she has a boyfriend, and if so who?

Remember that if you tip them of in any way the abuser might go escape and hide with the girl or even worse decide to get rid of the witness by murdering the girl. So you can’t just do the easy thing and ask the mom nicely.