T-Mobile lets you use your own GSM phone on their prepaid plans (perhaps their postpaid plans too, but I've personally been using their prepaid service since 2005). He can buy an unlocked smartphone and never pay for data.
Given that the backup phone isn't in the best condition, I went for a Samsung Monte slider off of Amazon. I get to test whether being a quad-band GSM phone is all I need for it to work here.