I don't think you would be a bad friend to offer to help at all. However, whether or not you should ask should depend on how close of friends you are.
You are basically telling her that her room doesn't look good, and you can make it look better. Yes, it may be the truth, but maybe there's a reason why they aren't fixing it to completion. Perhaps they don't have the money for the rest of the materials. You certainly aren't going to pay for those, right? I'm guessing that you will do the work for very cheap or free, but materials should be paid for by them.
If she mentions something about the room's condition, or has before, then you can offer to help. But otherwise you might be insulting her or making her feel bad.
I'm not sure what you meant by her parents getting the wrong idea... but I'm sure it's basically the same as I wrote above.
Oh, and if you two are really good best friends, yeah, just mention it. Best friends can get away with that sort of stuff.


