I think it's quite obvious she wants to be your "friend" so that she can check up on you and, if possible, dig for dirt.
There is no good that can come of this. The drama from "you won't friend me, wahhh!" is nothing compared to "I saw a picture of you with a beverage in hand. You must be an alcoholic! You can't be around my grandchild!"
Considering she posted a picture of her holding a drink up to grandchild's face as though he was getting ready to drink it and some little quip about starting early, (trust me, this was screenshoted a printed off for "insurance" purposes) I seriously doubt this would be an issue.
But do not worry! she's just sitting in my inbox, just chilling and that's where she will stay.
Though BF did something that might cast him into e-hell (or at least purgatory) when he was exchanging recently he asked the ex "So your mother sent my GF a friend request on Facebook, do you know whats up with that? It's kind of weird." Well she kind of got a deer in headlights look followed by a bunch of stammering and "uh well uh uh." So something tells me ex-MIL isn't the one that actually sent the request, or that it wasn't the one that actually.
Now this exchange should tell you just how bright she is. Any person should of been able to come with a version of "oh you know she's older and new to Facebook and doesn't understand Facebook etiquette blah blah."
Like I said, real piece of work.