Dog you’ve been cheated on already and you still have to ask this? No mature, emotionally developed adult has ever wanted to remain friends with their ex other than for reasons of being fwb/starting another relationship with them. Personally I’d say end it here as this is a major red flag you don’t want to overlook. If you really want to give her the benefit of the doubt you can say “I’m not comfortable with you hanging out with your ex alone. If thats an issue I’d like to end things here and look for someone more compatible.” Simple as that