its understandable you were tricked and trickle truthed. You wanted to believe your mom wouldnt destroy the home. you were 16? Her actions seem pretty clear that she knew she fucked up.

You have to admit to yourself your mother lied to you back then, and given it sounds like she met him again, she will lie to you now. Fool you once, shame on her. Fool you twice.....

I dont know if telling your dad is the right thing. Will he be able to handle it, will it lead to divorce, marriage counseling? The truth about her infidelity from five years ago will come out. Dont blame yourself or accept anyone else's blame, your mother did this.

Take pictures of the messages. need evidence. Would you rather confront her alone again, or drag dad in, its up to you. We dont know your family, lots of character traits that could affect what is the better option.