Lot's of good ideas here!
I just wanted to say that, from my own personal experience
Pediatricians CAN and DO miss these kinds of problems.
Parents CAN and DO 'stick their head in the sand' during these early months and years.
And, for me, I was the parent actually worried and asking about different subtle issues, and was repeatedly met with the "Don't worry", "Your overreacting", being treated as an obsessive hypochondriac parent, etc... etc... etc...

(My child has now been diagnosed with a disability)
Here are my thoughts, based on what you have posted.
This child is still very young.
Is it not true that a nine month old 'preemie' can actually be like a 7 month old?
I think their are milestone moments, like 'at one year old' at that first birthday, and like, when they start Kindergarten, etc.. And that it is at these points that many parents may really stop to take a good look at their child, and see that it does become very obvious if their child is having issues.
I would sit tight for a while.
Perhaps the parents may come-around if need be, and you will not have possibly ruined your friendship over something that would have worked out anyway.
After the child turns one year old, and the parents, and the pediatrician, still show no signs of concern. Then maybe you could reconsider the situation.