Sorry but idk what religion y'alls church is but doesn't the 10 commandments apply? Lol sorry just had to point that out, but i would just leave it at where it is, if its ur moms friends house she should be able to do what she wants, pls don't take that as being rude :)
You can ask your mom rather to move and make up an excuse or something. If your mom let her friend live with u guys then Ur mother can seat some rules to make it more comftable. You should tell your mothers friend that what she is doing is wrong and if she doesnt stop with ehat she is doing then yes you can tell your mother. I would deff not like it if I know my friend is sleeping with a married man, people who do that dosnt have respect no mather if they have feeling for him or not, it is wrong and it is braking a family. People are disguisting who sleep around with married people and the married person is a betraytor and a distructure of his or hers family
They had the nerve to bring you into it, know your not at peace within yourself, don't let there mistakes and lies make you feel like a bad person, you haven't done anything wrong. Talk to the friend, your mum shouldn't need to be involved. Goodluck
I believe it does have something to do with you now, I mean why would they make you promise not to tell? Thats just asking for trouble, if they wanted it to be such a secret they should have done it when you weren't even home, that's just putting pressure on you. I absolutely hate cheating, I wouldn't even tell your mum, I would confront the friend you live with, she should know married men are unacceptable, imagine if she was his wife.
Id say it has nothing to do with ur mom or u really so no need to tell