Mexico without a visa?

For non-RV reasons I was interested in the answers. We have a neighbor, who is a great guy, but got into trouble when he was younger. So jobs with dental benefits are very limited to someone with a felony. Anyhow, his teeth aren't in the best condition (no, its not meth mouth) and after reading some articles about folks getting good dental care in Mexico, I wondered if we could be RVing in Yuma (for example) where he could get his teeth fixed and recuperate with us on the US side, and then drive/fly back home. Apparently he only needs a passport to return to the US, so I was hoping someone would have a story about if we met & escorted him back at the border and he had as much ID as possible, that it possibly could be done.

Anyhow, that was my reason for following this thread. Thanks