As far as the emblem, display it if you're proud of it. The rest of you shouldn't have a sports team emblem etc if you're not on the team. Just as stupid of a comment.
X2........ It is okay to be proud of a certain branch without having served in it. I have AF tshirts I got from friends in the AF. Should I not wear them because I wasn't in the AF?
If he is representing through typed or spoken words that he was in the AF that is one thing but to put up the emblem is no big deal.
I can see your point but I see where the other guys who are in the AF is coming from, if everyone put up a military that they liked then we wouldnt know who was in the military and served.
I always wanted to be a Marine but I ended up having a child and getting married shortly after, even though I wanted to be a Marine and the Marines is my favorite Military I still would not put up the emblem under my user name for respect for those who actually did serve. Just my two cents.