I have 2 friends, friend a and friend b.

Friend a: known him for the best part of 10 years, pretty much grew up with him. Got some mad memories with him and a strong bond. His parents have split and his mam lives next door to friend b with new husband and family. 

Friend b: haven't known him as long, only around a year and a half maybe 2 years, great guy, confided a lot in him as I trust him and he has confided so much in me.

Friend b used to (not sure if it happens anymore) have regular sex with friend a's mam who is married.

I have known for a while. I chose to keep it a secret from friend a. Still is a secret.

Would I be the asshole if I told friend a what happened between friend b and friend a's mam even though it has stopped?