Upvote:3
You could consider JetBlue - their tickets include one free checked bag. However they don't have direct flights to San Francisco, so the savings in luggage are probably more easily offset by buying a direct flight, frankly :/
Also, depending on the terms of her ticket, and if you used a travel agent, you may be able to amend the original ticket to go to San Francisco. This way it'd all be on the same ticket (handy in the event of problems) and you may be able to get the baggage allowance included. It may not be on the same airline, but code-shared with one of their partners, if they have any that fly up there.