Should i tell her to stop or act like i dont care

If you care, don't try and act like you don't if you're in a relationship with her, because you'll end up cracking eventually.

You can talk to her calmly and communicate with her without being beta, needy or unreasonable. You don't need to force her to stop talking to him or emotionally blackmail her, tell her how you're feeling and if she is serious about the relationship and about you then she will respect that.

We don't know much about your relationship, how strong it is, duration, history etc. So there is not much to go on. However this is how i would handle it.

I would firstly sit her down, and ask her would she feel comfortable with you and a friend with benefits still being friends. If she says yes, then say you are okay with it, however there are certain boundaries. And then you guys should together decide the boundaries of your relationship. For example demonstrate your walking away power if she breaks any of those boundaries, or explicitly state you feel disrespected(if you do), and you won't stand for that.just an example.

Good luck
S.E.E.D

I do not agree with the advice above. I would suggest not talking to her. Setting boundaries only encourages deceit and covert behaviour.
Simply be ready to replace her if she crosses any lines.

Yea, be passive aggressive as this guy is suggesting, that'll get you far....

You need to write more. What is the nature of your relationship and theirs?

There is nothing passive aggressive about simply having standards for the people in your life and then sticking to them. If the relationship is not working for him anymore, he simply finds someone more compatible with him.

What does not work is sitting someone down and giving them a bunch of rules to follow, or "laying down the law". It's at this point where you change from fun awesome sexy guy to moody killjoy. You think any girl is going to tell her FWB "my bf says I can't talk to you anymore" and then think better of the OP for it?

If he doesn't like a girl that talks to her FWB, find one that doesn't...however the best thing would be to not worry or care about it at all. Maybe get yourself a second girlfriend so she is competing for you as well.