Then insert it. If that doesn't help, either
- either sell him a new printer matching the new cartridge.
- or offer to send it to HP so it can be repaired. However, if it's outside warranty that probably is more expensive than buying a new one.

If it's inside warranty and a new cartridge doesn't help, HP will repair or replace it.

I assume you read the manual about what to do when the right printer cartridge LED is intermittent? That should even convince the customer.

Kees