[GadgetKen]

Good news is AT&T still has those plans (and a new $25 plan) and does offer both regular and micro-sims. Their prepaid service is called GoPhone. https://www.att.com/shop/en/wireless...paidplans.html
However, smartphone users need to purchase a data pack on top of their monthly prepaid plan.
Other options include resellers for AT&T that offer different plan options such as Red Pocket, Straight Talk, H2O, etc. or a dual number UK/US sim chip such as ekit SimpleCalling, Tru, Telestial Passport, etc.
Easiest solution might be for your wife just to walk into an AT&T store in a city she is visiting, and have them install a GoPhone sim chip, activate it, and tweak the data settings. Even if there's a minor charge to set this up for her, it would be worth it just to walk out the door with a working phone with voice, data, SMS and MMS all working properly.
Or it is possible to buy an AT&T sim card in advance over ebay or from various online dealers.