You are trusting this other woman with your wife's heart and body, and yet she's not trustworthy enough to put a hold on a prospective affair while attempting to work on her relationship with her life partner. Your wife is getting into a vulnerable situation with someone who.s proving that she values pleasure over honesty. Of course you're concerned. I would explain this, and ask them to hold off. Another way to think about it -- infidelity sometimes makes people go crazy, crimes of passion and all that. Do you really want to risk a jealous, grief-maddened, betrayed husband putting a bullet in your wife if he catches them at it? Why can't they just wait???
This isn't cool and you are within your rights in not wanting to be a party to it.
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.