Check the date and time on her PC. I don't think that activating a subscription and then removing OneCare would trip the flag that doesn't allow the trial to be installed again, but it might.
If you activate her copy to your subscription, you can always remove it later. If any of your PCs are configured to be a hub in the Circle, you can "remove this PC" from the list of PCs and it will be unsubscribed.
-steve