Recently on FB, a certain person started popping up as a "suggested friend". Someone known to my family for years, but they'd never come up before like that so I went digging around a little bit. As it happens, this girl is my daughter's best friend, and also friends with 2 of my other kids. My daughter had posted a bunch of summer fun pics and this girl who is her best friend, commented (a lot) liked tons of these pics, because she was in most of them, or there at the time, etc.
I went looking and sure enough, several of the articles I found did indicate if someone you're not friends with, is suddenly very active on an account your are friends with, this is one of the "filters" in the alogrythm that FB uses to suggest friendship (common friends, common demographic, interests, etc.). I don't know if this is actually true (and trying to pin down technology is way above my pay grade) but in my case, that made perfect sense and I actually watched it happen.
In THIS case, it would make sense if SM's account was commenting and liking things on BM's account (since his social media seems to be the only constant in your DW's & friends case), that she normally wouldn't "like" or comment on - according to family and friends that found it out of character for her. But I imagine her profile, being from IN originally, and the same city as on BM's now deleted FB profile, roughly the same age, etc. is the reason her profile popped up to HIS friends from their hometown.
I hope that made sense & entirely jmo
(and now someone way smarter than me will come along and tell us all that's not at all how FB works, lol. So still, jmo.)