I did not have a ground strap until I built my own computer. I never had problems working inside computers (doing replacements, etc.) without a strap. I also had to work on a carpet (I had no choice) but it's an extremely low carpet (weave, but no pile at all).
If you have to work without a strap, remember to touch the chassis very frequently and don't shuffle your feet around. Also, pay attention to what you wear...don't wear silk, wool, fuzzy sweater, etc.
You forgot to tell the op to make sure the ps is plugged into a properly grounded outlet and installed in the case...THEN he may touch it. No pluggy inny = NO groundy.