I know how you feel cause I fell for the same kind of guy and I'm glad you found about it so early into dating him . Kick him to the curb and let him understand that he is a piece of shit.
I see what could happen if she did it. Guys like him can lie to you like nobody else. It would be this scenario . She tells him that she knows, he starts to make excuses, sound veeery truthful ,she starts to doubt her own doubts , says to herself "mm let's give it a try, maybe i was wrong" and bang it's too late... She shouldn't tell that she knows, she should make him suffer a bit and then leave without a word so he stays there without answers like a piece of potatoes .