So your friend can walk him without problems, but he acts out with you?
Our trainer explained that it's generally not a good idea to say "it's okay" to try and calm a dog. She used the example of Uncle Max coming to the door and the dog barking and growling. You say "it's okay" to let the dog know Uncle Max is not an intruder. What the dog understands is you're speaking in a calm tone and saying "it's okay," so it must be okay to bark at Uncle Max.
Perhaps unintentionally, you are sending the same kinds of signals when you meet another dog. If you tense up when another dog comes around, he certainly will sense that and take his cues from you.
Sounds like maybe the attack on Zach was more traumatic for YOU.
Maybe you should practice walking near other dogs without ANY reaction from you (verbal or nonverbal). It sounds like Zach can learn to overcome this.
Good luck!
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Linda
Remy -DOB 6/2/92; white male w/brindle patches;
Bailey - DOB 10/16/99; flashy brindle female
Shannon - 1992?-12/7/99 Flashy fawn; We miss her!