Please ignore u/MotherOfS6uirrel. This idea is simply dumb and not how adults should act. 

First of all, you don’t own her nor does she own you. Out of respect you two choose to be in a relationship. The most appropriate action is for you to discuss boundaries for your relationship and discuss how the  urgent situation is making you feel uncomfortable.  

Your responsibility is to share your feelings with her. She then has a choice to find a way to validate your feelings or at least explain, but it’s not really fair for you to just tell her, “stop talking to that asshole cuz I don’t trust him. If you don’t, I’m leaving”. I’m hoping you’re better than that. We have to show up to the conversation with our partner and allow them the choices. 

I really don’t think it’s any of your business or anyone else’s who she talks to unless it is actually impacting you in some way. Is she ignoring you? Is she canceling plans to go hang with him? Is she acting rude or distant to you? Has she given you any reason to think she is cheating or going to cheat? If the answer to all of these is no then simply just have a candid convo with her on how the whole thing makes you feel. It’s probably important to communicate you respect her individuality and don’t want to tell her what to do but you just want to share your feelings. This allows her to respond to you will respect, and you actually earn it in this situation. 

If this is a boundary (a hard limit in a relationship that would trigger action by you to leave or something. Usually abuse, or cheating, or something similar) then make it clear. Make sure you’ve communicated that you want to have say-so over who she talks to and when...  you know how that sounds. A better boundary would be, I don’t want to be in a relationship where my partner is lying to me about talking to an ex or is meeting up in secret or something. 

You deserve to be in the type of relationship you want as long as you’re not trying to control someone else or deny someone’s consent.