If he has a Texas DL and has maintained, and lived in, a Texas residence for 18 months, methinks he meets the requirements for dual residency. In that case he falls under the provision allowing for individuals with dual-residency status to buy or sell a firearm intrastate (within the same state) without processing the transaction through an FFL.
See items B12 and B13 -
http://www.atf.gov/firearms/faq/faq2.htm#b11
Summarized, it says that a person has dual-residency status if they maintain a home in two states and RESIDES in both states for certain periods of the year. Simply owning property in another state does not meet the criteria, but as long as the individual "maintains a home" there, they may purchase the firearm during their time of residency. As I read it, there is no restriction on what a "home" is as long as you actually live there (i.e. - a rented apartment vs. a purchased home).
From the BATF(E) page -
Q. What constitutes residency in a state?
A. The state of residence is the state in which an individual is present with the intention of making a home in that state.
Note that it doesn't say "own property" it simply says "make a home" which, to me at least, implies that you have a domicile owned or rented for the purpose of living there during a specified period of time over the course of several years. If someone rented a place and lived there one day a month, a good case could be made that he/she was doing so to skirt the law. But in a situation where the person has lived there 24/7 for 18 continuous months AND has a DL from that state, I see there being no question about the residency requirement being met.
If your buddy does NOT have a TX driver's license then I would not tempt fate. Run it through an FFL. Also, your buddy may want to go ahead and get a TX DL. I seem to remember a TX state provision that says if you are moving here with the intention of staying for more than a specified period of time (30 days rings a bell) that you are required to get a TX DL.
I'm not a lawyer. This is all speculation on my part so YMMV
Brad