Plebes can ride in personal vehicles with parents, officers, 1/C sponsor family "siblings," sponsor parents and a couple of other specified cases. Unrelated folks or girlfriends/boyfriends, not. They are supposed to take public transportation otherwise.
I recommend they just do the paperwork via the online app and just specify his name. That way, all will be legit, and they can get a FONA (friends of the Naval Academy pass) to drive on and pick him up.
http://www.usna.edu/Sponsor/application.htm
The only risk there is that if he wants a full-on sponsor family relationship and a home available to him on the Saturdays he's free, are they willing to be that available?
Now, do the powers-that-be stick their heads in car windows out in town and ask if the civilians he's riding with fall into an approved category? No. Do powers-that-be go to the same movies and restaurants as plebes, and do some feel inclined to report infractions? Yes. The plebe will then have to be very careful how he talks about those folks and how he got to see them in front of others and realize he's gambling a bit. There will be plenty of opportunities to break rules, intentionally and unintentionally, over the 4 years, why rush?
He can certainly walk out in town and join them for lunch or dinner in DTA. He can be in their home if he follows the rules about traveling to and from - paying a taxi seems awkward, but could be done.