OK knox blame it on the west....

anyway do you feel obliged to talk to him.... would it be appropriate just to stand to one side?

Maybe your mum knows but chooses not to acknowledge.

I am worried that if you confront the issue you will force action and then your dad might leave your mum.

Please try googling advice about 'midlife crisis'..... he probably doesn't love the girl but maybe using the concept to help him through a life change.