My mom is cheating on my dad

So my mother is having an affair. She’s never loved my dad and it’s a long story. To make it short they only married because they got pregnant with me. All of us children are grown now, me being the youngest and 23, moved out and everything. They have no reason to be together anymore, and my mom wants a divorce and has said she considers them ‘separated’ but has taken no steps towards filing an actual divorce.

She’s leading my dad on so she can continue mooching and using him, and he is naive and still wants to be together. I love my dad and it feels so wrong knowing she is cheating on him and he is either completely oblivious or in denial.

He has a hard time expressing his emotions and is quiet. I hate seeing him depressed or with false hope since he wants to stay together.

I don’t want my parents together, they honestly should be divorced but like I said my mom won’t actually do it because she wants all the benefits of being married to him while having her cake and eating it too.

What would you do??

I feel like I shouldn’t interfere since Its not my business even though she’s made it my business, but I also feel protective of my dad and feel like she can’t keep doing this to him. I’m so conflicted please help with any and all advice