A personal protection dog should be bonded to the person who expects to be protected by that dog. It is not a good idea to loan him out to your friends and expect him to do okay.
(That is why police departments with K9 units always have one handler for a dog, and do not let other officers mess with the dog.)
Yes, you can ruin your dog's training by treating him this way.
Here is a list of what I consider the five best dog breeds to train for personal protection. The German Shepherd is on there, but at the bottom of the list because of the numerous health problems that afflict that breed: