How much say do friends actually have?

There comes a time in every circle of male friends when one of the guys has a girlfriend and some or all of the remaining group members despise her. I know I speak from personal experience on both sides of that situation. I have been the guy who had the girlfriend than no one liked and more recently I had the unfortunate pleasure of being the one who did not like a friend's girlfriend.
Now the question at hand is how much should a friend do to make his point clear that his friend and the girl he is currrently dating not be together. In my case, I did not feel that the two people were compatible for one another. My friend is a hardcore romantic who is searching for that one person that can be in a serious relationship with him. This is what he is looking for in a girl. The girl, however, was unsure of what she wanted but she knew she could not offer him the serious relationship. She was not at that point in her life. Now, from the start of the relationship I was adamently against it and I made that clear to him. However, he continued to pursue the relationship and it did not end well. In fact, it ended after a few short weeks. I figured he had avoided the ultimate heartbreak that would have come from this doomed relationship that he even he said would end. Although, he stated that he was willing to work and work at it in the hopes that it could ultimately lead to what he is chasing. I figured it was my duty as a friend to protect him from the heartbreak that could stem from this relationship.
A couple of days later, the girl apologizes to my friend and says they should attempt it again. I knew my friend would go for it because he's just that kind of sappy guy. Nothing wrong with that at all, though. Anyway, this is where I felt I should try to become involved. The rest of the group and I heckled him incessantly hoping that would change his mind on this relationship. It failed. He continued to see her until Valentine's Day and I still tried talking him out of it. Sadly, that Valentine's Day ended in heartache as they split up again. The relationship is certainly beyond repair this time.
Now, I feel that it is the duty of a friend to protect his closest friends from an evil woman who can take advantage of him. My counterparts argue that while my actions are noble, I should allow my friend to make his own mistake and learn from them. I do see their point of view, but it does not make sense to me. In the process of falling for a girl, a man loses all of his logical viewpoints. It is the responsibility of the friend to be that objective, outside opinion to help him out. Nobody wants to see their friends get hurt.
Your thoughts?