Which I feel is incredibly excessive (especially considering most students do not want to sit through 2.5 hours of class anyway).

It is not, if i am late to my job routinely by 10+ mins every time i will get a stern talkign to after 2nd time and official warning after 3rd and some preprecussions any subsequent time.
It is a job you took it up, if you cant manage time well enough to get on time then sadly consequences happen, you being close to her does not stop her behavior from affecting others, even if it is an elective they paid for it etc.

What msot likely will happen from personal experience of similar thing in uni, dean will talk to the person and they will either get a mild admonishment or most likely some schedule adjustment if her tardiness can be reasonably explained.