I would remove the board and spray it down with a garden hose to see what that does. Try a scrub brush and some soap. Maybe a light bleach solution if necessary.
Check for cracks at the fulcrum and replace the board if necessary.
Replace the bolts with good stainless steel bolts, washers and nuts. Don't use the commercially available diving board bolt sets because they are junk and will rust as you can see in the existing set. You might have to cut the existing bolts to remove the board.
Check the nuts holding the base to the ground.
Note that the hardware holding the base to the ground might be in bad enough condition that you might not be able to reinstall the board due to safety.
Any time that you even touch a diving board, you take on a lot of risk and you should carefully consider whether you should even touch it at all.
Advise the customer of the potential complications.
Note: Just touching the board confers a certain amount of liability. If the board ever fails, an attorney would probably try to argue that the board was "Professionally Serviced" and that makes you liable.