What to Do?

What if you found out that your common-law partner of 20 years was HIV positive? And he swore up and down that he was not gay, that he did not cheat, that he has no idea how he contracted this illness. And yet, at the same time, did not want you coming to doctor's appointments with him, and you could not discuss his medical condition with him, or anybody else, and especially not his family? And maybe, because the only way you could rationalize how he would test positive, you figure that at some point in your relationship he must have had a blood transfusion and forgot to tell you. He's terrified that you will leave him, and yet by staying you know that you will never have sex with him again, because you are afraid you might also become HIV positive. But you love him like a brother, so you agree to stay with him.

Then, two years after his diagnosis, you start to think about other men. And you meet someone who you find interesting and attractive, and you meet up thinking that maybe you could be friends with this guy, but nothing else, because you would never want to betray your partner of 20 years. So one casual lunch, turns into drinks and dinner the next night, and the next thing you know, the sparks are flying. And it makes you feel alive and desireable again.

What do you do?
1) Do you end it right then and there, when you really haven't done anything wrong, continue living in a platonic and comfortable relationship, and keep your promise not to leave?
2) Do you be upfront and tell this new guy your situation when he asks (because he thinks you are single?) and hope that at the very least, he will still want to be your friend? But maybe you can get away with having an affair?
3) Do you end your living arrangement with your partner and tell him that you are interested in pursuing other relationships, but that you will still be there to support him should he need your help?

Someone I know is in this situation. We discussed all of the above options. I know what I would do, if I were in this situation, but I'm not her.

What would you do?