During that call, you need to push for all the info about the LED lighting. What brand, model, who hooked it up, how it was hooked up, etc...
Without this info, you could be spending a very long day there figuring it all out yourself. Before you even get to the troubleshooting part.

If the customer kind of blows it off as unimportant (how people can do that blows my mind), ask him if he wants a bill in the hundreds or thousands of dollars because you had to find all that out yourself - when he already knew.