Unlike ATT does in many cases, T-Mo still won't force you into a data plan if you bring in your own unlocked handset. All they want is your money for voice service, generally.
If you plan on getting her a subsidized handset via the T-Mo site, you can easily see which ones require the data plan.
Also, an important thing to keep in mind is that if you buy an unlocked handset, quite often it will not include the correct 3G frequency bands for use on T-Mo's network. T-Mo utilizes 1700MHz for 3G service in the U.S. I am assuming she isn't as concerned since she won't want a data plan. In this sense, most of the quad-band GSM unlocked phones would likely suffice.
I am looking for a phone for my girlfriend, however I'm a little lost as she has t-mobile and I have att. Basically all she wants is a phone with a physical keyboard and a decent camera. She can't have a phone that requires a data plan (can't afford it), but I'm not sure which ones do and don't require one. I was looking at unlocked phones, would she be able to use an unlocked phone and not need to get the data plan? Or if I get her an unlocked phone will she not be able to use it if it requires a data plan? Any help would be great, thank you!