I'm in a similar situation in that, I'm an adult but I keep my horse at my parents who also board 2 other horses. I am also the most experienced of everyone there.
Here's my thoughts on your situation. So you have the feeding / getting out issue covered. Good.
Now for the girl. If that girl were at my parents place, you can bet everything you own that I would be making sure my PARENT's investment was being protected. By that I mean, I would make sure my parent's had a WRITTEN agreement prepared BY A LAWYER releasing them from all liability were this "boarder" is concerned. Good family friend or not, when push comes to shove your parents will be held responsible if something were to happen.
AFTER that is in place, all you can do is step back & stay out of the way. As much as it is going to drive you up the wall to watch & not do anything ( TRUST me!!! It. Drives. Me. INSANE!). The only time you can step in and say anything is when it directly affects your horse (or your mom's).
Then again, if they aren't paying for anything, I may be inclined to go the "if you don't like how we/I want things done safely, feel free to leave at anytime..." route. It's not like your parents are making any money letting them board there.
Actually, I'm sort of leaning towards the "get with it, or get out" mentality at this point.