Why would any thinking, experienced adult disable the thumb safety on a pistol?
But,
A P01, the standard P01 CZ has been selling for years doesn't have a safety. It has a lever that "decocks" the hammer from full cock to half cock. Pulling the trigger after that well make the pistol fire.
An Omega P01 (a newer variant CZ designed/sells) can have ambidextrous decocking levers or safeties.
There were a few (as compared to the other two models) steel framed compact P01 pistols with ambidextrous safeties.
The first two above are aluminum framed pistols, the steel framed one is a semi rare pistol.
If the kit included a CGW hammer then the issue may be that a shallower full cock notch in the hammer doesn't allow the sear to move into proper position to provide clearance for the lobe on the safety shaft to move under the forward arm on the left side of the sear.
It that pistol is a steel framed P01 then all that is required is an adjustable sear from CGW - or slowly filing metal off the bottom of the sear arm or the lobe on the safety shaft until the safety shaft can properly rotate under the sear arm.
If it's a decocker/aluminum frame or an Omega P01 then I have no experience modifying either one and no idea what might be wrong.
But, back to your question. A lot of members here have modified their pistols and I don't recall any of them saying the safety didn't work when they got done and that was okay "because everyone else does it."
I wouldn't go to the range with your friend. His safety attitude needs a serious adjustment.
A CZ75 Compact (this one was a standard Compact with only a left side safety lever) sear cage showing the forward arm on the sear and the lobe on the left side safety lever.
green arrow points at the arm on the sear, red arrow points at the lobe on the safety shaft that rotates back under the arm on the sear to keep the trigger bar from moving the sear and releasing the hammer.
