a friend of mine/yours/his/hers/them = один мой/твой/его/её/их друг = один из моих/твоих/его/её/их друзей (the article "a" means "один" here, though we know it is not translated, sometimes it is.
my/your/his/her/their friend = мой/твой/его/её/их друг
P.S. Antonio, don't hesitate, please answer my PM message, because it looks like I offended you. I would like to arrive at some conclusion.
