Here’s the link for the road tax rules which is easy to find on the .gov website.
https://www.gov.uk/government/publications/ins216-how-to-apply-for-free-disabled-tax
The motability rules are based on these rules as they can’t allow you to use a car which breaches them.
The same issue has been previously discussed on this forum.
Someone having the car and just saying it’s available to the disabled person isn’t enough to meet the rules.
As others have stated its easier to confirm with motability.