If you are going to go landside anyways, then I'd say either meet in front of the BA desks or meet at the bottom of the Fairmont Hotel Entrance (airport hotel) at the far right of the US check-in area if you don't mind a bit of a walk.

But you could also meet in the BA lounge if you want to do bypass Canadian customs. But doubt the lounge has that much to do if you are talking about a 5 or 6 hour transit.