He looks purebred to me, but I'm no expert. However, as far as one GSD looking like another, well...you've been on the forum. You should show your friend some of the photos here and just how much variance there is in the breed
I have a "typical-looking" black and tan that everyone recognizes, but then I also have an older long coated female that people swear is mixed. They don't look much alike, but they are both GSDs.
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Jack - male GSD b. 8/9/10
Lillian - female Weimaraner b. ~2005
Mattie - female GSD b. 10/18/02
... For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack ― Rudyard Kipling