
What you must remeber that to him this lady was acting suspiciously, as she was nervous of him, and Dobes are natural Guards. This is where the socialisation is so important, as they learn what is a normal situation! You letting people in after the door bell rings, and command of that'll do. Not lunging at people in the streeet, as people walking the street is normal every day occurence. Same with you meeting and shaking hands with a friend, this will be seen as social greeting, and not an attack on you!
The key is your response. If he takes a spook like this, act normal and upbeat, ignoring his reaction, but ensuring he obeys leave command, and that he come and sit and say hello.
I had a wary BSD bitch, but primed to the "sit" "say hello" command she was quite happy to great people and be patted, but without this promt and reassurance from me that it was OK, she would not have accepted the person trying to touch her and would back off!