Videotaping someone without his consent is just creepy.
Footfaulting yourself to see if the footfaulter will "get the message" is just misguided.
If you want to let him know, just tell him that you think he footfaults sometimes and he may want to have someone check his feet when he serves. If someone told you that you footfault, you would probably want to get other opinions or have someone watch you when you serve to confirm it.
Footfaulting is like being pregnant. You either are or are not. There is no kinda.