I don't know how easily your grandmother travels, but what I would suggest is standby travel. . . . .

Ask the AC Agent how full the later flights are and if they look "open", book the ticket on the flights with Aeroplan seats - with the return on a later day. I believe you can then standby for an earlier flight on an earlier day with your aeroplan ticket. . . I have done it in the past.

Before your grandmother heads back to the airport check the loads for your flight again (call AC and ask - they should tell you).

Worse case scenario is a hotel night in NY, but that should be cheaper than a full fare ticket.

Just curious, What day is she flying and to what NY airport ?