Just found out that my mom is being unfaithful to my dad help?

I recently just found out my mom is sleeping with my friends dad instead of my dad (I saw them cold stone creamery together being a little too close for comfort, well, I guess they seemed comfortable, but thats besides the point). I really don't know if I should tell my dad cause it'd be weird for me to tell him, or if I should like blackmail her to get stuff I want (she won't let me stay out past 10 or wear Cannibal Corpse shirts cause they're immoral, lulz, what a hyprocrite :( ). I think my dad would be mad if he would out I knew and had been blackmailing her instead of telling him, but if I tell him I doubt he'd give me a reward (I took pictures with my google phone for proof) but I deserve one right cause this would cost a private detective a lot of cash.