Entering Canada isn't the problem. Returning to the USA is the problem. There's a very good chance at this point that she would be stuck in Calgary and unable to re-enter the USA -- and her return flight to the UK is out of SFO, several months from now.

If she was flying from the UK directly to Calgary (much as she plans to go directly to Montreal next year for Worldcon), there would be no issue. It's the US transit that's the problem. Must keep our country safe from them dangerous furriners, you know. *fume*