Not my field... at all.... but a few things quickly pop into mind.
- If my Judge Judy viewing experience is accurate, he would need pictures of the actual scratching in porgress as well as video of the person doing it.
In other words, pictures of someone walking near his car and then disappearing from view - would be of no use. Probably would even be detrimental, as it may cause your customer to jump to conclusions, get pissed, and go asssail the person, if it's someone he suspects is the culprit. Anyway, I can't think of anywhere *inside* a car you could mount a camera that would give you the angle required to catch someone scratching the sides.
- Supposing you could find an angle to mount the camera; wouldn't you need multiple cameras, to catch the multiple views a person could approach from?
With 1 and 2 in mind, I think cheap and reliable are ruled out; not to mention who knows what would happen to a CCD in a sealed car on a hot summer day. For this reason, I would guess that video tape is also ruled out, forcing you to go to some sort of digital storage solution.
I just don't think there is a practical way to do it, especially if cost is a factor.
Now, on the other hand - if the vandalism is occuring when the car is parked on or near his home - it would be easy and inexpensive to monitor the car 24x7x365, and have court admissable evidence.