I'm going to ask my friend to stop dating our mutuals
Why don't you want your friend dating your mutuals?
There are so many other people in the world. All it does is create drama.
While normally it is not good to tell somebody who they should date, it is okay to tell them that you are uncomfortable with a situation. It will after all affect you and your other friends.
What you mean it's normal to tell someone who they shouldn't date? That's just stupid, I can do that if I want and it's nobody else business.
It is ok to ask people to stop if you find it uncomfortable but you need to respect they have wishes too. Maybe you can't work out this friendship and will need to find a new group of friends.