score:3
There is no reason why you can't mail or courier the SENTRI card to him. It's important that the US Department of Homeland Security, which issued the card, is aware that he lives in Mexico, but if they're not willing to ship the card there, but will give the card to residents of Mexico, I don't see any legal impediment in forwarding the card.
You may want to send the card with a tracked service, although, interestingly, the U.S. uses ordinary untracked first class mail to mail trusted traveler cards to Canada (from personal experience with NEXUS).