I agree that in your situation the mother keeping close tabs not only isn’t likely to happen, but probably wouldn’t do much good. If my nieces or nephews posted something inappropriate, I’d let their parents know. Again, that probably wouldn’t do much in your case, but that’s the first step I’d take.
As far as people posting inappropriate stuff, I use the delete button. If it’s posted directly on my wall, I use the delete button and ask the person who posted it not to do so again and remind them that my family, coworkers, etc. can see my wall, as well as my kids if they happen to be around when I’m logged in (I’m not quite as bothered when it comes to private messages). If they don’t take that into consideration after that, I set them to ignore or unfriend them. It doesn’t happen often, but I’ve run into a couple people who take the attitude that if my kids see what they post it’s my fault and they’re not going to refrain from posting vulgarities and adult material. Those people didn’t get a second chance.
Getting back to your niece, since her mother isn’t concerned, why not approach her father? He’s clearly a bit more concerned with her moral upbringing if the mother feels she has to hide this stuff from him.