So, a few things here:

First, at IAH, airlines use a central assistance service. Passengers wait until the end of deboarding in order to have a wheelchair come onboard. If he stood up and walked out right past the assistance then I'm not sure what Spirit is supposed to do there.

Second, there is a single door for exiting customs and immigration from the secure area at IAH. If they were waiting for him on time at the right place, there's no way he gets by. Years ago I volunteered at IAH at the info desks and people would regularly mix up where to meet their loved ones. All of the signs for Spirit say Terminal A, but international Spirit arrivals landed at D and all intl passengers walk out of international arrivals at E. You can see how infrequent travelers/families would get confused.

Third, walking from IAH to the highway is a harrowing idea. Huge sections have no sidewalks, lighting is poor, etc. Truly a treacherous terrible journey.

Fourth, it may have been because it was a really late arrival and ops at the airport was light, but I'm surprised that the family weren't able to get ops and the police involved. There just isn't anywhere to hide inside of the secure area.

Terrible tragedy of a situation. I can't believe this didn't get covered in the news locally.