How about you tell them WHAT they're doing wrong and WHAT they should be doing in place of it, instead of outright telling them they suck at Dota, which will achieve nothing? Everyone sucks compared to someone who is more experienced/better.
I started playing Dota 2 a long time after my friends had been nerding it out in WC3. I started playing HoN with them for a while then we headed over to Dota 2. It helps a lot telling your friends about tactics / hero play styles and the likes, instead of just raging at them.
Now that me and my friends are on a equal level it tends to get more rage related though
thing is, you can only tell them so much, i cant constantly be looking at their character, or playing over their shoulder, its already bad enough i have to constantly direct him. Its just to that point where its almost better i coach the guy, which is laughable. Getting caught out, or ganked is all on him when you say there are 3-4 people missing on a map. Its so many different factors that arent being grasped which seem rather basic, and when not done are so detrimental.
I havent outright told him anything, i try and be nice and give advice, but its like a lost cause everytime i play, so sad.
I think im just goin to start doing ranked matches, and leave it at that. Once the mmr gap is there, he'll notice it himself.
Most recent match, same friends. Nothing different, same mistakes as always. This game they blamed it on the juggernaut, it was his fault he was split pushing and not teamfighting, which caused them to be caught out and killed in unbalanced teamfights.(they are the shadow shaman and Sand King)
http://www.dotabuff.com/matches/1082379459