Assuming that all of these foreign nationals have valid passports, there are some other considerations; The US citizen could just walk out of Mexico and return to the USA. The remainder would have to have US visas in order to do the same. Lacking that, they would have to fly out and they would encounter the need for their FMT at the airport in order to board a plane to their home country. Rather than turn in an expired FMT, I have heard of folks claiming to have lost it or to being ignorant of the fact that they were supposed to keep it.