Well, your gunsmith is definitely wrong. Now the question is what to do about it. He probably won't correct the problem if he won't admit there is one. There are several on-line instructions and videos on that pistol but the ones I have seen miss one critical point. They take you through re-assembly to a point, then leave you dangling.
The trick comes after the gun is all together except for installing the hammer and the safety (the safety shaft is the hammer pin). At that point, grasp the gun just like you were firing it, holding in the grip safety and pulling the trigger. At that point, the hammer is easy to install and the safety shaft can be inserted to hold it. And you are done.