Can a supervisor stop an employee from contacting other employees outside of work.

Hey all, this is on behalf of my sister - She works for a cafe and often texts the next shift to bring items from one location to another to restock. Yesterday the manager asked the other employee if they text outside of work, which also includes non-work related conversations.

My sister suspects the manager will pull her aside tomorrow and tell her that it’s not allowed, however to my understanding there is no way they would be able to stop this.

The only actual framework I am aware of is the Right to Disconnect Act, but that doesn’t stop employees from contacting one another.