If they do not receive a higher rate mobility allowance they are not able to lease a vehicle on the Motability Scheme, however if their circumstances change in the future you they become eligible. In the meantime the following links may be of interest to you.
Department for Work and Pensions
For enquiries about claiming PIP or DLA allowance:
Call them about PIP on 0800 121 4433
Call them about DLA on 0800 121 4600