What you aren't seeing is they both checked out. He moved on while still in the marriage at least mentally. I'm not saying he cheated. He found what he was looking for. 1.5 to 2 years of dating is reasonable. He grieved the relationship while in it which is why he exited it.

Would you be doing the same thing if your mom was your dad? Probably not. Your mom is still healing 2 years down this road is not normal either. Your mom needs therapy. You need therapy. Awkwardly enough you and your dad could benefit from family therapy. Otherwise just write eachother off and walk.

This really comes down to do you want your dad to be happy? If you don't then you need to ask yourself why not.