Next time you have sex with someone, please use protection against STDs, your sexual health, and that of those you sleep with is really important. There are some really nasty diseses out there. You probably know this now after catching one, but it just had to be said.
If he was cheating on her, you may not have been his only other sexual partner.
If I was his partner, I would want to know that I needed to protect my sexual health. It would be better if he told her, but if he refuses to or can't be trusted to, yes I think you should tell her. Obviously, you need to tell him about the STD too.
I understand that it is interfering in their relationship, and hope you wouldn't do it out of revenge or to try and break them up, certainly you don't want him to leave her for you (you want a relationship with someone who won't cheat), but I think that since her health is involved it is better than not telling her. It also isn't as if this is the first she has heard that he might not be completely heterosexual and faithful, so is unlikely to be as bad as the first time she found out.
No need to go into details about your relationship with him. Just the facts that you had unprotected sex with him and have tested positive for X and believe that she should get herself checked out.
I realise my advice is against the trend of the thread, I don't know if there is a completely ethical solution to this but if it was my sexual health at risk, then I'd want to know. (I'd also want to know if my partner was a liar and a cheat, but hey ho).
OP, have you finished the tests or do you need to go back to get tested for HIV and Hepatitus after a number of months? Hope you are okay and you stay protected in future.
No idea if my post counts as a boost or an argument, maybe its a bit of a lecture. But seriously you are worth more, both in terms of your health, and in terms of having relationships with people who aren't liars and cheats, who are free and able to return your affection.
Best of luck whatever you decide to do.