All the advice given above is good..but you must let her know her guy's situation and have her get tested,assuming she doesn't already know. I would imagine that the longer she stays ignorant of the situation, the more difficult it would be to treat her, thus making her situation more desperate.This way she gets the proper diagnosis and correct treatment.
But above all, stand by her regardless of the outcome as true friends.
It's not hard to make decisions when you know what your values are.