You can't test a triac circuit dimmer with an ohmeter. It's not a rheostat, it works by chopping the a.c waveform.
I don't think a digital multimeter will give any valid results when the dimmer is powered up either, you need a scope to check the waveforms under load. Bench test it as Paul says. If it's u/s, get the customer to buy a new one.
If it works ok on the bench, he's got wiring problems.
