I used to work as a 'trade plater'. A large amount of my work involved collecting Motability vehicles, at the end of the contract period, and taking them to auction. I had to complete detailed paperwork about the condition of the vehicle but there was nothing on the forms which asked about whether the windows had been tinted. I would very much doubt that service agents have any questions about this on their paperwork either. So it's likely that the car could come to the end of it's contract, and be taken to auction, without Motability ever finding out that the windows have been tinted.
Motability is possibly the most disorganized and inefficient organization I've ever encountered. They frequently don''t even seem to know who's got what car and how long their contract has left to run. So it seems highly unlikely that they'd normally be bothered about such a modification.
However, I must point out one important reason why your friend might need to notify Motability about the tinted windows. If he has the 'full package' from Motability, he'll get his insurance through them. Anyone who has the windows of his car tinted must inform his insurers because this counts as a 'modification' to the vehicle, which almost always raises the insurance premium. Failure to notify the insurers can invalidate the policy.
Chris