Thought you use to be in the Royal Navy, come on get it right its a lamp not a bulb. Bulbs grow in the ground :lol1:
Firstly what kind of lighting is it?? Is it Neon or is it florescent lighting they have. Big difference. If it is Neon then it is high voltage and you can’t touch it with out being qualified. If it’s a lightbox then it is probably florescent and will have tubes and starters. How long has it been there as you can sometimes try and help people out only for it to backfire on you. You can change the tubes and the starters yourself without having a 17th edition qualification but the problem comes when a few months down the road something goes wrong and they expect you to fix it for free.
I would tell the customer that you will have a look at it and replace any existing tubes & starters that need replaced but can’t offer any sort of warranty or better still ask your local spark to look at it.