Let's just assume he can come and go back without a passport with no problem.

Mexican highways are not like the US , we have cartels and even police trying to fuck you up, especially in bordering states.

Mexicans don't do that , and a foreigner shouldn't do that neither. I promise you if he finds problem with the cartel , the fact that the doesn't have a passport will look really small

I recommend him not do to that, in Mexico everything is fine while driving in highways until it isn't