All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.
From a distance here, not seeing the details of the application, indoors or outdoors or the characteristics of the burner, can only give a limited opinion.
At first glance however, if it has electronic ignition and with that of course a flame sensor, and that works, and tests well for flame failure shut down, it should be fine.... however the local codes may have changed. and I cant see the other details from here.
To protect your customer and yourself, you might consider asking the gas company to send a man out to inspect one of those and get a detailed invoice from him showing what he checked.. if he had no recommendations to condemn or modify the installation you have that degree of protection.
You could also ask the city plumbing and heating inspector to look one of the jobs over, at some risk, but if he had no complaint you would have even better protection.
If these units are inside there can be potentially more issues than if they are outside.
With the wiring, pilot/ ignition/ flame sensor at 15'from the gas valve there is no issue...high tension wire needs to be fit for the spark ignition, same type that it uses now most likely.
the gas line at 15' should not pose a problem either, you may want to upsize the gas line to the next size up from the gas valve to the burner head, depending on whats there now, how many 90 degree els it has, how it has worked so far .....and the btu rating of the burner.
Tell me if its propane or natural gas. and if the pots are indoors or outdoors. Let me know what you think, and what your concerns are... we can go from there.