Question

So there was this one time that a relationship I was in ended. It was hard for me but I had other good friends. One of them said to me "good I never liked him anyway, he was SUPER boring." Well I thought, and actually still think, that he's hilarious. But never mind that. I couldn't help thinking 'why the heck didn't you tell me that during the year plus I was with him? Why did you so willingly come to our game nights and dinner parties? Seriously?' 
Question. If a good friend is dating someone boring, or irritating, or a little too self centered do you tell them? I'm not talking when they are a horrible person, or you think your friend is in an abusive or controlling relationship. That I think needs to be discussed. I mean when they are a great person but being around them makes you want to rip your arm off and throw it at them. Me, I'd want to know. But apparently people don't tell in the spirit of good friendship.