If you trust him then you should let him tell her that it will never be anymore that is if he really loves you , only wants you but in all the same basis do not stress it but do be aware of the out comes that may come , already their , if the ones that are already there do not feel right to you then leave him because as you know that people in the female gender that I have noticed are very convencing but he may not fall for it , if it feels or seems as he is then say its eaither her or me , if he can not truely answer it then say nice knowing you , leave him.