I agree with Pomtzu. if it's on Facebook it's probably going to get back to mom one way or another. How old is the girl? if she is a minor then I'd definitely say you need to notify the mother ASAP. if she's not a minor then thats a little trickier but I'd still bring it up with the mother even if just to give yourself some peace of mind.

Good luck. I know how this feels. My neice is really making the worst possible choices and now she also has a kid to look after when she can't even look after herself and I see the things she posts on facebook and I can't help but wonder how long until CPS takes taht child from her because she is just totally unfit mentally to take care of this child... at least the way she presents herself on facebook that seems like the case. but her parents didn't do anything about it when she was a minor and living under their roof so now they definitely can't do anything about it now that she is 18 and out of the house (for now)

I wish people were more aware that what you put on facebook is PUBLIC and anyone can see it no matter what your privacy settings are.