Client Interact with another client?

hello every1
Well im trying to make a “Trade system” between 2 clients.
what im doing is : when client 1 press the trade button it it should send the message (widget) to the client 2 form him to accpet or ignore. Feels pretty simple but im not understanding how can client 1 can interact with client 2 so so it can set the widget (message) visibility of client 2 to visible. How can i say is that player who needs to set the visibility of the widget to visible?
Thank you in advance