While she should ask the airline for confirmation, here is how it should work.
In Winnipeg, her checked baggage will be labelled with a special sticker identifying it as a US connection. She will check her luggage in Winnipeg for the flight to Calgary.
In Calgary, she will proceed to the US Pre-clearance section on the departures level where she will have to retrieve her checked baggage. THere is a US Luggage Transfers station just outside of customs where her bags will be delivered. It is not delivered to the regular baggage carousel (special sticker). She must retrieve all her luggage and proceed through US customs. After clearing customs, there is another belt, staffed by airport personnel for her to put her checked baggage on so it can be put on the plane. Finally, she will clear security again and go to the gate for the flight to Seattle.
A couple of notes: Airline staff will be fairly familiar with this process, so there is no harm in asking for directions. Make sure that all your business is done before clearing customs, because there is no easy way out. The biggest pain in my mind is having to clear security again, which means no liquids. If she buys a bottle of water in Winnipeg, she has to drink it all or dispose of it by the time she gets to customs in Calgary.
That's it. It can seem a little complicated the first time, but the simplest way to think about it is that when you get to customs in Calgary you just have to pick up your bag right outside customs, go through and re-check it right after.
