Once she cancels, all medallion perks go away.
If you were upgraded before cancellation, you will likely be downgraded. It did not used to be this way, but seems to now be this way.
I suppose that you could do online check in, and then she could cancel, which would give your boarding pass Sky Priority stamped on it, and you could then use that check in and security line. Print the boarding pass and don't show it to SP check in in case they would take it away and give you a new one.
However, if you are already upgraded and do it this way, you risk being downgraded to a bad middle seat.