when you look at a friend through the view friends tab, you can't attack, at least not for me, same thing if they send you something, but a guy who was trading some stuff said he could hit them if they sent him something, is this possible or was he full of BS?

You cannot attack your friends unless you find them in the battle tab.

If someone is in your legion, they can send you something through the legion tab.
If they leave the legion, you will still see them linked in your news feed and can target them through that link. If they are not also your friend, you can attack them.