A friend is having trouble keeping up with the class. You suspect that bad study habits may be causing her grades to drop.
The first thing I would do would be to talk to my friend and see if their was a reason she was having trouble keeping up with the class. Maybe something is wrong at home that she needs someone to talk to about. If nothing was wrong then I would ask if I could help her. I would teach her the things that she doesn't understand and review everything. I would also help her change her study habits by showing her how I study to get a good grade in the class. If her grade didn't improve by me helping her, I would suggest that she talk to the teacher and stay after school one day to get help.
That is what I would do if one of my friends was having trouble in a class.