They had clearly veered off the PanAmerican Highway (which is a bucket-list trip from Mexico to Chile) to take in some other sites east of that route, since that specific highway doesn't go near Veracruz. There are several 'don't miss' areas and sites they could have been heading for in southern Mexico, however my inclination in July would have been to get out of the sweltering lowlands (including the Yucatan) and head to the mountains, particularly San Cristobal. It would be a 5 hr drive from Coatzacoalcos. But I know nothing about their interests or heat and humidity tolerance.
JMO
JMO