Welcome to Mamapedia, G..
I'm sure this is very difficult to hear. I'm not sure why the parents told you, given how upset you are. But them being furious isn't going to help anything. They need to have a sensible dialogue with their daughter, and get her some reasonable and unbiased health care.
You are the grandmother, and as much as you love your granddaughter and want the best for her, you have to stay out of this entirely. You say nothing. This is between parents and child. You don't stare at her or look askance at her. You have to let this girl know that she is loved, and that her family is a source of support for her, a place for her to come when she has questions or problems or fears.
Try not to treat her any differently at all. The surest way to drive a teen into the arms of a teen boy is to reject her, rebuke her, or let her think that she's somehow tainted and no longer worthy of your love.