Can you cancel the trip and still get a refund? If I were them I would prefer the honesty and bravery up front and delayed gratification on the trip versus being blindsided by tension or maybe even an explosion while I was already there. It's not dumping on them to just let them know you guys are going through a rough patch right now. Poly doesn't work without loads of comunication and honesty, so if you think it's a path you might want to go down at some point, why not get a head start on the tough but necessary components now? Again, if they're good friends they'll respect you more for honesty, not less.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.