Congratulations.
Honestly, I think you should just rip the band-aid off and get a headstart on your life with her.
You should both just sit him down and tell him you love one another, and that you know about his infidelities. Throw in a "We just know you've been unhappy for a long time, and we connected through worrying about you."
Then, hopefully you guys can all remain friends and still take camping trips, cruises, and road trips together.
Good thing she's not already pregnant.
That'd be a lot of news to handle, right?