The XX4X pistol series was designed for a revolver like long trigger pull of reasonable weight. Essentially, it's a 15 shot revolver.
Without getting excessively complicated, it's possible to smooth the trigger pull and slightly reduce the weight and overtravel, but the weapon can't have a short crisp trigger without major work. That is, converting the pistol to a traditional double action.
If there's a circular plug in the side of the slide, the plug could be removed and replaced with a safety. Several other parts appear to be necessary also. However, there are 4 versions of this pistol and it would be best to have your friend consult with Smith and Wesson about exactly what he wants to do, what's necessary and the cost. He might be better off to trade that pistol for something else.
BTW, your friend needs to realize that the only effective safety device lies between his ears. "Accidental" discharges are due to mechanical failure and extremely rare. The usual cause of negligent/unintentional discharges are having a finger on the trigger when it shouldn't have been. The only time a finger should be on the trigger is when the weapon is on target and ready to fire.