I don't think there are really any hard and fast rules about that
If your mate knows that both you and your cousin are bi, and doesn't have a problem with either of you being bi, I'd predict that he probably wouldn't have a problem with knowing that the two of you hooked up. Well, not a serious problem anyway. Like, one of my (very close) female friends had sex with my brother a couple of years ago. I wasn't angry or upset, I didn't have a problem with it. I was initially a little taken aback (but that was more to do with the fact my female friend was dating a girl at the time xD)
I suspect you may be making mountains out of molehills. If one of my friends randomly came up to me and said "I can't keep this a secret any longer, I can't stand the guilt; I hooked up with your cousin a few times ages ago", I'd find it a little odd that a) they thought it was such a big deal and b) they randomly confessed it to me after so much time.
I suppose there is the possibility that your mate might find out (from his cousin, or someone else) that the two of you hooked up, and he might wonder why either of you didn't tell him at the time, but again, I'd be surprised if he had a problem with it.
(Bearing in mind, this is my perspective as an almost 25 year old poly guy with very liberal and flexible attitude to love, dating and relationships and so might not be representative of your experience
)