Well, the simple version is that it is his relationship to deal with.
What you need to to is to protect yourself. If that means using condoms with John, then insist on that.
Be John's friend if he wants someone to talk with. He needs to learn this lesson for himself - he's an adult and has the capability to learn. It's not your job to protect him from his own worst instincts.
Learn about Herpes - many folks have it, and it just takes special precautions to reduce the risk of being infected and deal with it if it has happened.
If the drama is rampant (which it sounds like it is), you have the right not to let it into *your* house, or even into your life. Let them get a hotel room to do what they want to.
Has John had a previous history or habit of being drawn to distrustful people and drama queens?
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb