I will talk to my friend and her parents to inform about the health problem she has to her teacher.then I will talk to the teacher personally about my friend's health problem which is a reason for her frequent absence in class which leads to her poor performance in class. I will ask the teacher to support her. I will tell that she hasn't spoken of her health problems because of her shy nature.

It's good that u want to help ur friend!
Okay!! I will say how to help him!!
U can help ur friend by;
Sharing the assingments done in school with him so that he can do the preparations of exams at home.
U can ask about his's health problem in details from him.
Take him to a health expert or a doctor and explain ur friend's health problem to them.
Provide him with all the medicines or other requirements pescribed by the doctor or health expert.
Lastly, do any help ur friend asks from u.
          I hope this answer would help u!!