The
Citi Platinum Amex is one of the best offers out there right now. Five thank you points for gas/grocery/drug and three on everything else for the first 24 months. Annual fee is waived for first year, $99 after. If you redeem the points for flights, you can pretty easily get 1.5 cents of value per point. 15K points bonus on signup too.
She would still need a second card for coverage where Amex is not accepted.