There's no immigration preclearance at YQR, so your friends will have to clear in MSP. I haven't flown through MSP in ages so I don't know how their system works, but I expect that bags will come up after immigration for customs inspection (usually random) and then they get rechecked. Security will have to be gone through again. That's how they do it in DEN when flying out of YQR. It's actually not as bad as it sounds. I do wonder why the routing is through YQR and not YWG-MSP. That's a funny one. I'd get your friends to check at check in to see if they can get a direct flight to MSP. In fact, I'm wondering who the YWG-YQR leg is on as only AC flies that route--a Delta stock ticket?

I've spent lots of time in DEN and though it isn't the most exciting of airports it is a pretty easy one to transfer in and kill some time in.