Someone not doing a mileage run would willingly do that? Yeah, bonkers LOL. Are the 5 days in Halifax and the 6 in Toronto with no time in Vancouver? Not sure how much they are going to enjoy their time in Halifax after flying 6000 or so needless miles. I wouldn't even do that for a free flight, let alone "just cheaper".
4 hours will be fine in YVR for the connection, with the usual caveat that the first flight arrives on time. Maybe they will be lucky and have the Air Transat flight leave LGW really late, the crew runs out of hours and they have to land in Halifax LOL.