CAUGHT MY 60 YR OLD MOM CHEATING

I'm 21 and my parents have been married for 22 years. They met at a wedding and started a family when my mom was 39 and my dad was 37, so a little later in life than most people. My mom always says she thought she'd end up alone. The thing is, my mom asked me to help her with her phone and I don't know why, but out of curiosity, I checked her Facebook messenger and found messages between her and a man from her past (like when my mom was my age or younger) and they have been exchanging messages since April of this year. They don't start out like "cheating messages" but about a week ago this man, who she knows because it is her sisters brother in law 🙄 basically professed his love... And my mom's response was basically how she liked him when they were younger and

he will always have a special place in her heart and yes she has a family but she feels similar and knows she shouldn't but she doesn't care that she does. And this really breaks my heart. I guess what I'm looking for is advice on how to approach the situation now. If she is unhappy in her marriage with my dad then she needs to address it or leave. I will not allow her to have one foot out the door. If anyone has any advice I would really appreciate it. I always felt lucky to be 21 and still have my parents together. Now I don't know how to feel and I can't even look my mom in the eye.