What did you do in your three hours with him? Did you do cross country procedures? If not, I'd say you're on the line for not checking him out before you signed him off for it.

I've flown a lot around DFW and found the controllers helpful and fairly forgiving with students. What are we talking here, did he/she clip the airspace near the edge or did he/she go cruising over the top of DAL?

Kinda sucks you got stuck with them because the other instructor disappeared. I'd have a good talk with that instructor about the student. Mistakes happen, but it's the regular instructor's job to catch stuff like that before sending them away with somebody else (or solo).

Mainly though, did you get a call from Regional or did the student get a number to dial? A lot would ride on whether that happened, and if so how it went.