You definitely need to ask him if he has discussed this with her. I'm sure/I hope he is responsible when dating as far as the physical end, but if their relationship get's to a point where it is emotional (for both of them) prior to becoming physical, she could end up leaving when finding out because he held that from her. She deserves to be told. If she really likes him and he is careful with his disease, there is no reason that the relationship couldn't work.