I have no idea if this is a possibility or not, but here is a possible scenerio:
The Michael isn't your husband, but the old girlfriend went looking up your husband and tried to friend everyone with the name Michael B or Mike B (I don't remember the fake last name you are using), and this Michael accepted her as a friend.
The reason I say this: I was looking for my best friend from high school who I haven't seen since the mid 90's. I know she got married, but don't know her last name. But thought, maybe I could find her brother. So I did a search for his name and a bunch came up. One of the pictures looked like it could be him (the last time I saw him he would have been around 16) but I wasn't sure. So I thought I would try to friend the guy and if he accepts me, that probably means it was him (because why would a total stranger accept a friend request). Anyway, the guy did friend me, but it wasn't my best friends brother (so I defriended him). It struck me as odd that he would accept a friend request from someone he didn't know, but apparently some people do it.
So, there is a slight chance that the girlfriend sent the friend request, thinking it was your husband, and the person accepted it, even though that person doesn't know the girl. And if this is true, then your husband wouldn't have a clue about any of this.
Probably isn't true, but it's a thought