I know lots of guys with DWI's (yes, some with more than two!) who have airline jobs. One guy got hired by a Major six months after getting it (yes he told them about it). It is NOT a career ender. But, it does not help you one bit.
Most people that I know who have them got them 5-10 years ago in college. The one that got hired by a major had not had a traffic violation, ever, until his little incident. I'm sure that helped a bit.
Tell him to go ahead and start the paperwork with Canada. From what a pinnacle guy told me, it takes something like 6 months and about $500 just to get them to let you into the country. You have to fill out some type of citizenship application, or something like that. I don't know about Mexico.
But all in all, if he was my student I would give him a speech with these main points:
1) Tell the FAA!! NOW!! You have to send a letter to OKC. A guy I knew who was CFI/II/MEI "forgot" to let them know. They took all of his licenses, literally, and a year later he still does not have them back.
2) From now on you drive like a frickin grandma. Slow and defensive. No tickets allowed ever again.
3) Don't drink. If you drink, don't drive, If you drive, don't be surprised when you are sitting in a cubical 9-5 because you can't fly anymore.
4) Tell him to go talk to a lawyer, explain everything, and see if he can get out of it. Are you in Florida? I remember seeing tons of "Got a DWI? Call me" billboards driving around. Tell him to call one of those worthless pos lawyers.
All in all it's not the end of the world, but he will have to strive to be the best. A lot of pilots drink like fish, so the interviewing pilot will most likely understand. It's the HR chick you have to worry about!