I have done jobs like this on several occasions, with mixed results. The performance of an owner provided fixture does not reflect on you or your professional installation, provided that you have installed it properly as instructed and to code. Your job was to provide the path and control for the power, and to hang this fixture, and that is it. Performance of said fixture is the sole responsibility of he who furnished it. If a warranty is required be sure to word it to not include coverage of owner furnished materials.
If they are sour about the results, and you would like to keep them as a client, write up an estimate to replace the fixtures and give them a deal to do it. If you are not willing to bend any, write up an estimate to replace them at what you think is a fair price for your time and money.
We have been doing a lot of retail lately and they mostly have owner furnished fixture packages which include lamps. I have learned to exclude lamps and fixtures from my warranty. The furnished it, they own it.
On a side note, HID lighting loses intensity over time, they may just need some new lamps, but be careful fixing their junk, you may end up owning more of this problem then you want.
