DBailey:
1. Do the classroom and pool work locally to you. Make sure the instructor knows that you will be going elsewhere for the certifications dives. The instructor willl need to fill out and sign a few pages of your logbook.
2. Schedule the certification dives with the shop in Mexico. SB members are a good resource, but you could also check with your dive shop for recommendations.
3. The certifying instructor has paperwork to fill out and submit. Some people will try to get you to bring back the information and have your original instructor finish it, this is wrong. The person you do the check out dive with needs to fill out, sign and submit the paperwork.
This is only true for a PADI certification. Other agencies, such as SSI, NAUI, YMCA, PDIC, and some others, participate in the Universal Referral Program. In the Universal Referral Program, the referral instructor fills out the referral paperwork, gives the new diver a temporary card, and sends the paperwork back to the originating instructor to obtain the permanent card.
Kwbyron, if your wife would be taking a PADI course at home, you probably won't have a problem finding a PADI instructor in Mexico to do the referral. In that case, the referral instructor in Mexico will take care of the card. If she does something else, make sure to follow the Universal Referral system and don't let some PADI instructor in Mexico insist on doing it his way. You do want to check with the referral shop before hand to make sure they will do it the way you want, either the PADI-to-PADI referral or the Universal Referral.
I did my certification as a referral and I have had lots of students do it that way. Damselfish is right, doing the cert dives won't kill the vacation. Actually, the skills only take a short time, usually at the beginning or end of the dive, and the rest of the time, you are just taking a tour and enjoying the critters. A referral is often a good way for someone who is interested in diving but not in braving cold water at home. Do some research, but don't hesitate over the idea of a referral. There are lots of very good instructors in Mexico.