Can this marriage be saved? Yes, I think so.
So, her first foray into poly, as a solo girl, not as a swinging couple, went a bit wrong. We're all human, we all make mistakes.
Maybe she doesnt remember if he came in her mouth because she was pretty stoned or tipsy? Just assuming there were substances involved, you damn hippies.
You gave her a very long leash, Podunk! A whole week on her own without her partner, in a party atmosphere (topless dancing!), lots of attractive cool people about. She might have sort of "forgotten" she was married in the midst of all the stimulation. (Not sure how young she is, but the brain isn't fully developed til age 30. There can be disconnects with reality in the teens and 20s.)
Added to that, with our culture's incessant mono programming, even poly people can feel guilt over extra-marital sex, especially newbies. You might want to read the last few pages of Carma's blog, when you have the energy.
I understand how upsetting and frightening and frustrating it has been for you this week to have to pry the truth out of her bit by bit.
Take a nice break this weekend, eat, sleep, try and relax a bit and then revisit the issue of expectations and boundaries.