We don't know this Houston witness was a stranger, or the circumstances around what he told to Rachel's mother Frances Langston further than what she reported to a journalist.
The part of the article we're referencing makes it sound like a stranger just happens to waltz into her home, then points at the Rachel picture saying "that's the baby I saw" recounting the moment he just happens to be in FW on the very day the girls disappeared. I think context is missing to a large extent, and knowing more about this reported witness would be valuable to weigh up FL's claim about what he told her years after the fact.
A. Who is the Houston witness? Was that witness followed up by police?
B. Why are all links to Houston in this case only coming from the letter's unknown author, Debra, Frances Langston or the father (by proxy of his treatment location)
So many questions.