my opinion: The heightist hypergamy that this woman viscerally feels has led her to come to the realization that women like herself will reject him.
It's tough out there for short men. Also, the definition of a short man has changed a lot since the 1990s.
White women in the USA are programmed to think anything below 6'0" is trash. 20-25 years ago, 5'9"-5'10" was more acceptable for White women in the USA. Not so much anymore.
I'm 5'10" and I was saved by a late growth spurt. I entered high school at 5'1". Reached 5'8" at end of sophomore year. Beginning of junior year was when I was tall enough to be taller than enough women to make dating worthwhile.
Prior to the growth spell, I experience some social problems due to relocations and being height deficient, I was a target for bullies in late elementary (after the first relocation) and junior high. Freshman-sophomore year of high school weren't bad in terms of bullying but was still experiencing the trauma of multiple years of bullying to due in part to height.
It's difficult to say whether the relocations or the height issues were the bigger reasons for my childhood bullying. I think it was the relocations.
The mother has reasons for concern.