There are several issues to be concerned with.
1) Will the body of the guitar support humbucking pickups? The bodies of some Strats are already routed to receive either humbucking or single-coil pickups. You can tell by removing the strings and (carefully) the pick guard. Hum bucking pickups are generally twice as wide as the single coils. You can see if the pickup routes are large enough for a humbucker. Some pickup manufacturers make "stacked" humbuckers that will fit in place of the normal pickups. Make sure that your friend does the research, though. Some of the stacked pickups are intended to reduce noise without changing the sound.
2) Matching the remaining single-coil pickups to the humbucking pickup. Humbuckers produce a much stronger sound than single-coil pickups. If you add one humbucker, the level of the other two pickups may be too low. Again, several manufacturers make sets of pickups that have hotter windings in the single-coils to match the level of the humbucker. Consider buying a set of pickups designed to work together, or even a pickguard that has pickups mounted and pre-wired. I believe that some of the sets are complete, including pickups instructions, volume and tone pots, etc.
3) Is your friend comfortable cutting wires and using a soldering iron? If not, take it to a tech. Even though changing pickups is not a difficult job, cutting a wire too short or burning the wiring insulation with the soldering iron can be catastrophic. If he decides to try the mod himself, caution him to make a good drawing, showing wire, control and pickup locations before he starts cutting.
4) It may be less expensive and stressful to trade the Mexican Strat in on a "Fat-Strat" that already has the matched pickups installed.
