I don't believe fitting or "a fitting" is a hobby kit you can hook up to the rider, turn the switch, and then it will work, elininating all problems and discomforts.

If a rider is complaining of numb hands, I would first try to reduce the weight on the hands. The most direct approach to that is to move rider center of gravity back away from the 'bars. The ideal case of this is to make the rider take on the tuck of a champion downhill skier or cross-country skier while in a glide. In a tuck over skis, your CG is above the feet somewhere. For a cyclist that might be too extreme. I don't know a general rule, in these terms. I do know that for me it is possible to have too much setback, in which case my butt tends to move forward on the saddle and the sitbones fall off the platforms - causes perineal abrasion!! So while I set back the saddle to reduce hand pressure I try not to set it back past the point where pressure reduces to an acceptable level.

After succeeding with this adjustment you might find you are reaching more forward than you like, in a trainer test or road test. At this point the stem could be changed for one with less extension, or the 'bars changed for one with less reach. If like me you have usually a 9 cm stem with long-reach MAES or Nitto 115 bars, you could get some nice compact-pattern bars to pull the grips closer to your body. Less drop, but, there are other solutions as well, such as keeping the original old-school long reach rando or touring bar (the MAES) and getting a stem that is a cm or two shorter. If you need a stem extension less than about 5 cm, parts availability can be a problem.