It is May of senior year. Presumably, she has weeks left of school. If they are giving her a path to graduation, take it and get her out of that place. Is she being allowed to participate in senior activities such as prom and graduation? Work to give her a different perspective.
Unfortunately, their behavior is not unusual with the private prep schools where my children were involved. We found big, suburban, strong public schools to be much, much, much better choices in dealing with illness, disability and diversity. I also do not find it unusual that students are treated differently. Is the other student absent for medical reasons or were they a one-time thing, like two deaths in the family, an extended family trip for an overseas emergency or several days off to deal with a special opportunity? The school may see illness as more of a problem because it implies ongoing absences. The other considerations are the other family’s status in the school and the possibility that the other student has ongoing absences for an activity that the school supports (a competitive individualized sport or some talent the student has).
I would also advise you to choose a college carefully and put any accommodations in place with the disabilities office before the student arrives on campus. One of my kids had recurring absences and illness and chose a small, top, liberal arts college. They dealt with illness very, very well. Another of my children has a learning disability and is at a competitive, private big university. They have given minimal accommodations and advising has been poor. I attended a large, state university and class attendance was not a big factor in grading although a student who just stops attending classes would have a hard time making it through exams. It was also really easy to go part-time or withdraw from semesters and maybe graduate in 6+ years. On the other hand, my kid’s small, liberal arts college had much stricter guidelines on how long students could take to graduate. Look into these things before she goes to college.