Your friends definitely should have skipped their scuba trip or booked it elsewhere. My family was discussing doing a cruise to the Mexican riviera and decided against it because of safety concerns.
Hopefully they return home safely and consider a different destination next year.
The San Antonio Express-News had a column, a few days ago, by a columnist of Mexican descent. A very elderly relative died in Mexico, but no one from his family attended the funeral. They were concerned about their safety in traveling to and from the small town in the interior where the funeral took place. Instead, they held a wake in El Paso.
I have a friend (also of Mexican descent) who works for an import-export company in the border town of Laredo. Part of his job is meeting with Mexican businessmen, who fly into Nueva Laredo, on the Mexican side of the border.
A few years ago, he would make an excursion of the trip across the border to pick up a client. Maybe do a little shopping. Wine and dine the client in Nueva Laredo. Now days, he says he fills his car with gas before he leaves, and the only stop he makes in Mexico is at the airport, either picking up, and dropping off, the client.