I would try to convince my friend to tell his girlfriend and make him understand why it is so important for him to tell. It's very selfish (and a little evil, in my opinion) for him not to tell someone he cares about that he has this incurable virus that could mistakenly be contracted and lead to death. If he refused, I would tell. I would want some one to tell me, whether they were my friend or not. So, I would do the same.
Caveats:
1) I would first try my best to get my friend to tell his girlfriend, himself.
2) I would likely find a way to tell the girlfriend, anonymously.....though if my male friend didn't really tell other people, he'd likely figure out that I was the one who told.
3) If he didn't tell himself, I'd likely be so disgusted with him that I'd have to distance our friendship. I can't say that I'd completely cut him off, but we definitely wouldn't be as close as we were. Why? He's not trustworthy.....and if he would lie about something so detrimental to save his relationship with his girlfriend, he would do the same to me.
4) I'm getting a little angry just thinking about this. :-/
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"Do what you can, with what you have, from where you are."
- Theodore Roosevelt
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People might not get all they work for in this world, but they must certainly work for all they get.
- Frederick Douglass