The car is not yours, you rent it from Motability and any damage not notified to Motability or their insurers breeches your rental agreement. Imagine you own a nearly new car and your “friend” doesnt tell you he has had an accident in your car, and repairs it with a scrap door, then you find out after weeks or months especially when its auctioned at the end of your lease,
They will not be happy to say the least, if he is your “friend” he will accept his mistake and make good with RSA and Motability, but you know this anyway. do the right thing