DS (1yo) has a name with a popular short version so we call him by the short version, let's say Chris. A friend assumed Chris comes from Christian and called him that, so I politely said "Christian is a lovely name but actually DS is called Christopher".
I thought that was the end of that, but the friend has spent the last year calling DS Christian. I have told him I don't like it, he won't stop, I have asked him how would he like it if I called his kid by another name and he said that was a wonderful idea, etc.
Yesterday was DS's birthday party. As we sung happy birthday the friend shouted 'Christian' over the whole group. He also went to another mum who did not know the origin of DS's name and told her a whole story about how we chose Christian because of a film, complete with quotes.
I am about to shout at this guy next time he does this and have asked DH to talk to him first as he is primarily his friend and he may get through to him. DH says he doesn't see the outrage, I am exaggerating and should let it go.