I have no idea what to do, do I tell her?

put your self in her place would you want to know

yes tell her she has a right to know and it wont be easy

Why not just wait until he's not around, get his phone and say "hey who's phone is this" in front of your mom with the phone in your hand... you can just say you saw it lying around. This way she will look at it, see the messages if any, and can deal with it her way. Otherwise you could talk to your dad, tell him you know, and that you want him to stop/come clean with your mom. Either way I think you need to involve an family relation/adult that you can trust in this to help guide you on moving forward and to protect yourself. Good luck and be careful!