get her one of these:
AT&T 3G MicroCell™ - Signal Booster - Wireless from AT&T
in the 6 years i have owned my home i have never gotten a signal inside the house. about a month ago the local att store was giving these away to people who lived in known bad reception areas, and i qualified. now i have 4-5 bars in every room. the only downside i have noticed is that there is a slight (1-2) second delay when using it. other than that it has been flawless. i bet if you were to call att directly they would help you out. they offered to credit me the full price plus tax if i drove 30 mins to the next att store when my local store ran out of them during the giveaway but i opted to wait the 3 days for it to arrive at my local store.
depending on her usage they also have an add-on plan that for $20/month all calls made or received via the micro cell are free, including if you were to receive a call and leave while still talking. as long as that call is connected it would not count against your minutes.