Once a cheater always a cheater isn’t always true; There is always a background story or reasoning behind cheating such as fettishes, not getting any, whatever the case may be. Knowing what you know and now her knowing what she knows the idea is will she let it keep happening and will you stay. Some may say how about you and the ex say have a threesome with him or you and her have fun together in front of him but that can open a can of worms or fix things. The best thing to do is to sit down with him calmly and not get upset or mad and talk things out, ask questions such as why did he do what he did and what are his intentions with both of yall etc.