ok kids heres the deal i got friend one and friend two (henseforth noted as F1 and F2) these two guys are room mates. F1 has been dating this girl for like three years. F2 has what is in my opinion an oddly close relationship to the girl. F1 and girl broke up for like five months durring which time girl slept with F2 as well as another less significant friend (that makes 2 of F1s close friends) everybody we hang with knows about this except F1 and he and girl have been kind of getting back together. the question is does F1 need to know that his lovely lady did his room mate. i dont know what the answer to this quandry is but im not looking forward to F1 finding out then asking me how i knew and didnt tell him. 
however, now that you're in the equation, this is a tough one. what does your gut say?
if i was in your spot, in all my humbleness, i'd stay out of the equation.....if and only if i was factored in, at which point...if F1 asked me, "why the hell didn't you say anything?!" i'd say straight out, this was between you guys--it wasn't my place to be involved.
....and then I'd let F1 stew. and if F1 was a real friend, he wouldn't stew for two long.
just my $1.00 - $0.98.
good luck......and thank u for saying hi on my blog!