If my partner insisted on meeting my other partners when he knew that is something they don't want, i would think that they have problems respecting boundaries and end the relationship.
It also sounds as if you're threatening to punish her if she dares reunite with this guy. What you need to realise is that the boundaries you request need to be about you and her not her and him. As long as they have safe sex, their relationship and how it doesn't work is their business, not yours. If their shitty relationship does start to affect yours then make sure he stays away from you and she doesn't discuss him but other than that, it isn't your concern.