What should i do? dilemma?

I've recently successfully proposed to my partner of 3 years. We're planning to get married in 5 months, however about 2 months ago I accidentally slept with her best friend while she was on holiday. It didn't mean anything - just a really bad drunken mistake, but now she has said that if I don't sleep with her one more time, she will tell my fianc�. Her friend is really hot but I'm in love with my fianc�. Do you think she will really tell her and ruin their friendship and the relationship between me and my fianc� (should I call her bluff)? Or do you think I should just sleep with her and forget about it (she has promised never to mention it again if I do - and I truly believe her when she says that)? What should I do?