They should meet a day in advance without telling him and come up with a joint strategy. Then, when he arrives, let him have it.
Also, at least once, have a junior staffer tell him that it is “disrespectful to come here and litigate this in front of the Canadian media”.
“Keir Starmer will also host European heads of government and the leaders of Canada and Turkey at a special defence summit aimed at presenting a united front on the Ukraine crisis.”
So….a defence meeting with Great Britain, Europe, Turkey, and Canada without the US. This sounds an awful lot like a NATO meeting without the US.
Trump may not have to worry about leaving NATO. They may do it for him.