<human>: key:
mum = my mum
dad = my dad
brother = brother
M = brother's friend
Y = brother's other friend
girlfriend = my girlfriend
S = girlfriend's friend
myself = me

So I live with my mum, dad, brother, his friend (M), and my girlfriend. My girlfriend's friend (S) is over almost all the time and so is my brother's other friend (Y). This means I live with 6 people, with 2 over almost all the time other than to go home and sleep.

Mostly my Brother, Y, and M hang out on one floor of the house and my Girlfriend, S, and myself are on another, and my parents chill in their bedroom mostly. 

S, my Girlfriend, and myself are all quite close. My Brother, Y, and M also close with eachother. Between the two groups of us we're not super close however, except that recently Y and S started dating. It's a rocky relationship but S is pretty insecure and really wants it to work, but me and my girlfriend get the impression Y doesn't seem that invested. So a few days ago M tells my girlfriend that Y is cheating on her, and my girlfriend tells me because she doesn't know what to do. I mostly trust his opinion partly because he's very blunt and crude with that sort of thing, and I've overheard them talking when I walk past their room on my way to the kitchen (they're kind of loud). 

My question is, is it better for me to tell S, for my girlfriend to tell her, if so do we tell her how we found out, or just not tell her at all? I'm really struggling with this decision and my girlfriend has been very anxious about it, especially when she has to talk to or deal with Y. I don't know if this is a common question online, but I thought I'd throw it out to /r/advice.

Can you condense the text into a few key points?
<bot>: close friend's boyfriend might be cheating, girlfriend found out from distant friend, told me; how should me and my girlfriend proceed?