Hi Moeman,
Just some more questions . Is the reward booked in econ or biz? If booked in econ, would they allow you to change the reward ticket to biz - or have you already tried that? Can you change at this late date to another partner, or is United not flying between the two locations?

I'm not sure, but if all else fails, what about a letter from your gran's GP or something stating that she needs every assistance to get home on the 25th? If she shows up at the airport with this, I really think a 70 year old lady on standby might arouse a bit of human compassion at Air Canada!

Good Luck!