1.HOW YOU CAN HELP YOUR FRIEND:
*You could help her find out in which subject or activity she has interest in and then help her to develop her skills more in that act. or subject.
*you should try to ask her to share her problems with you,or you can talk to her parents and ask them to encourage her in her studies so that she doesn't feel inferior among other students.
*Try helping her out in studies and see where she's facing the problem.
*if the main problem is about her health ,then you should advise her parents to go see a doctor.
*you should also try developing her language and communicating skills.As sometimes children are not able to express their problems or share with other people.
2.STEPS TO BE TAKEN OUTSIDE THE SCHOOL.
*usually nowdays,the main issue that bothers such children is the society,what the other people think about them.Your friend might be facing this problem because of her health issues.so i suggest you could make her feel comfortable around you and your friends.
*you should try working on her social skills in order to help her deal better with the environment.Or you could convinces her parents or brother/sister to help her deal with that.