Out to the property to do it, rather than just give the guest permission.

I guess he could lock it in a closet, have it connected to an extension cord that runs under the door or through the wall to an outlet outside the closet. And then if it ever needs to be physically restarted, just instruct the guest to unplug that cord from the outlet he can access, and then plug it back in.

That allows him to restart the router without having to physically touch the router itself and prevent the factory reset.