This is usually a power-related problem. Ask the customer if he is using an AC adapter to power the keyboard, or if he is using batteries.
If the customer is using an AC adapter, then ensure that he has the correct AC adapter that is intended for use with his keyboard. If he is using the incorrect AC adapter, then the keyboard might not be receiving adequate power. In addition, use of the incorrect AC adapter could cause damage to the AC adapter and/or keyboard.
If the customer is using batteries, then suggest that he try changing to a fresh set of batteries.
If the customer has tried both battery power and the correct AC adapter, and continues to experience this problem, then the keyboard would require repair.