Just refer, but write the form to accurately reflect your knowledge of the pupil. Then when they won’t assess it’s not *you* that the parent perceives as the barrier.
Implement any reasonable adjustments that would be offered if the child had a DX and monitor to see if they make any difference whatsoever – make parents aware you are offering this “to see if it helps” then report your findings.
When an ADHD diagnosis is made the first thing recommended by NICE guidelines is a parenting course (or so I was told when my daughter was diagnosed). You can offer something like this, if you run it, on the basis that this is what you can offer that gives parity to a child with a diagnosis.