So I was in a somewhat similar situation with my ex fiance at the time. Found out about a kiss (she told me) with my best friend and I was okay maybe that was it no problem I will let it go since everyone was drinking. Just found out this last Saturday (after 9 years) that apperently when I fell asleep in the livingroom with everyone else (like 8 or so people) she gave him a blowjob 3 feet from me while i was asleep. So from this situation all I can say is people will lie to cover themselves and only give the bare minimum so not to be caught. Best bet is to just accept you wont get the whole truth and if you cant handle that (you shouldn't anway) then you should leave em.