found out my mother is sleeping with her best friend's husband.

My father left the family about a year ago but my brother and i still maintain contact with him. My mother only communicates with him for the occasional grocery run. Just recently, i found a pack of condoms under her bed and i also found texts that suggest she's been sleeping with her friend's husband. I was about to bring this up with my mother but i just didn't have the heart to do so because, to be honest, i haven't really been being as supportive as i should be since my father's absence. I'm scared as to how she'd react if i told her i knew. Obviously i want them to stop but i don't know how to approach this situation. A close friend advised me to speak with the husband since i told him that i felt i couldn't discuss this with my mother. Would that be the right thing to do? I was thinking of warning him to stop this immediately or i would out him to his family and ruin his life.

I also think that this relationship has been leading to her put her anger and frustration on me because of my father's absence and the guilt of having this secret relationship. This is all however a guess.

A close friend advised me to speak with the husband since i told him that i felt i couldn't discuss this with my mother. Would that be the right thing to do? I was thinking of warning him to stop this immediately or i would out him to his family and ruin his life.

I hope this is the right subreddit for advice and if not, I'd appreciate if somebody could redirect me. Thanks