Yikes! That's a horrible decision to have to make. I had that same problem with my daughter when she was 14. She had horribly heavy periods due to, what we now know, was a thyroid problem. Anyway, she was on a low dose of BC to ease the flow and we now feel it was a mistake. She became very depressed on the drug and had nasty mood swings. Since it didn't happen for weeks after she started the drug, it took us a while to figure out what the problem was.
Also, since hormones play a big role in autoimmune disorders, it's scary to mess with that for fear of stirring up more problems. Some people with ITP also have antibodies that can cause blood clots, and birth control raises that risk.
However, if the bleeding is bad, you don't want her to become anemic, so you have to take that into consideration.
What are her counts?