Whats the context of this question? What thing sets the bool and why should the server use the client value?
Normaly you would make a replicated function. Thats a custom event. You can set the replicate to “Run on Server”.
This is called by a client and runs on the server. So you could make an input for this event (bool) and pass the bool. Use a “authority” node to make sure the client only calls it. Then on the server side of the event, use the variable you passed to set the variable serverside.
A custom event always has a red node and a blue node. If you create the event, you get the red node. Here you set the replicate option and add inputs. Now you can search for the event like every other node and use this second node (" blue") to call the event. So the begin play goes into a delay node then into a athority node, then on " is authority" into a set node that sets the replicated bool directly and on the client exec you take the custom event and mark the bool input with true. At the other end of the event you take the bool and again set the bool.
That should be all. Cant give you pictures because im on phone.