They can't be much of a "friend" if they damned him for wearing a Poppy in remembrance of all his fellow Irish men who perished in the First World War.
That was just a thought. He should do what feels right, but i would think that he has or had told many people that he would never wear one. If it was me in that position i would wear it. But I'm not from the North and didn't grow up in Derry so i would not presume to know what that must have been like.