I'm not sure you'll even be able to do that... The last few times I've checked with AAgents, the system is much more strict now about making sure that the name on the credit card matches the name of the AAdvantage account being redeemed.

I think it'd certainly be worth asking AA about how to handle this: given that Grandma doesn't have or want to use a credit card, how is she supposed to redeem her miles? (Assuming she's also not into PayPal, etc.)