Just like any restaurant, there are good Mexican and bad Mexican restaurants. IMHO, the good ones have
great service
a full bar
great tasting, fresh tortilla chips
good, fresh salsa, not too hot
food that is closer to authentic Mexican than Tex-Mex
a few very unique authentic Mexican entrees in addition to the standards
the servers must have a basic ability to speak English
a cozy atmosphere, not too loud. Kitchen noise must be isolated from customers.
The restaurant biz can be brutal. Suggest he quickly advertise and run specials to build a clientele. If he waits much linger, it may be too late. Also suggest he visit many other Mex restaurants in town, especially the ones with full parking lots and see what they have to offer.