if you are not her manager, or in HR how the hell do you know what her sick notes say. Your company is very unprofessional if it gives coworkers details of other peoples sickness, or indeed pay, bonuses and other matters.
If you are her manager or in HR then why are you asking this question?
Anyway, it is up to the company what kind of sick leave they give people, and how long it is before they satrt asking her to see occupational health (or do you know all about her consultations too?) o start asking her to meet with them about whether she will be able to continue in her job. In short, yes, she can be asked to leave if by nature of illness she cannot carry out her job, but only if the company/organisation want to do it.