Yes, I think it would be betrayal if she isn't aware. He should inform her of your past relations and explain the situation. Or if he doesn't want to introduce her to swinging yet, then I think it would be wise to get a hotel and not be tempted to jump into bed together. No sense in ruining what the three of you had or the two of them have. Just explain to him that out of respect for his current relationship, you feel it's best to stay in a hotel to allow him some privacy (that sounds better than saying so that he doesn't cheat on her
)
And to answer your last question, yes. I would hope that he would tell her of your past. However, I think it's his responsibility to tell her, not yours.
Good luck.
~SS