Brief background - my particular group of friends is very, very close. I am slightly on the outside as I don't see people as much due to being a mum and not being out too much. Recently, two friends (let's call them A and B) split up after a 4 year relationship / living together. A (man) was totally devastated and felt this came out of the blue. Haven't seen B since that point but have since learned that she is sleeping with C - One of A's closest mates. A doesn't know this (yet) and the only person who actually knows is D, C's flatmate. (sorry if this is getting confusing). D has a history of depression and is very affected by friends / relationships etc. He is very close to all of them and now has had to put up with B and C round at his place all the time. He's asked that they not flaunt it in front of him and not bring it to the flat but is getting increasingly upset at just how blatant they are all being and seem to be ignoring him. He's said to them if they feel this strongly to wait 3 months until the dust settles but they refuse. He's kept this to himself until today when he told me (I had guessed tbh, as they were always very flirty / friendly at parties and often found alone together)
D feels like he is totally betraying A by letting them carry on but at the same time they are his friends too (and mine, although I am more distant from them and obviously not having to live with it) I should point out that it is less than 3 weeks ago that they split up and are currently still living together as they have a mortgage. Both myself and D suspect there was something going on before the split.
It would totally divide our friendship group if this came out and we both feel tha's a huge sacrifice for what is most probably just a fling. I am also worried for my friend's state of mind over being forced to carry such a secret (he crashed his car the other day as he was crying)
I just don't know what to advise him or what to do.