You have to switch the network selection in settings I had the same issue using the public mobile while in Mexico was very inconsistent until it wasn't
Try this. Ask him to go into Settings, then Cellular, pick Network Selection, turn off Automatic and see what options come up. I've heard Verizon is pretty good along with AT&T. Tell him to click on one of them and then reboot the phone. This should automatically connect to the one he picked.
I know in the US you sometimes need to manually choose a network like at&t so try manually going through whatever network that are available and see if you can get one to work. As I didn't know what one pm uses in Mexico
Try selecting different Mexican networks to see if he can get service. Also try resetting network connection and rebooting the phone. If that doesn't work, he should message a customer service agent. In order to save time and unnecessary messaging, he should provide his account number and the last four digits of his credit card.
