How do you tell a friend without losing the friendship?

How do you tell a friend that the man she is in love with and wants to marry doesn't feel the same way about her? I have a friend that I've known for many years (since middle school) and she's been dating a guy for the past three years. They've been fighting recently and the other night they had a big blow out. She came to me and asked advice, giving me her side of the story (there's always at least three sides to any story). At the time, I suggested they get a 3rd party involved (such as a relationship councilor) that was completely objective. She said they'd had a few of their friends act in such a way. But that's not the same as getting professional outside help. The next day, her boyfriend spent an hour and a half bearing his soul to me (essentially) and he talked about why their relationship is the way it is and why he feels the way he does about her. He's told her all of this. MANY times. But she doesn't seem to get it. He has outright said he's not in love with her and he's not going to marry her. But because their relationship is mutually beneficial, he's not going to break up with her or kick her out. Essentially, his feelings for her are completely platonic. How can I help her to understand that? She's my friend and I don't want to hurt her. But I also don't want to see her wasting her life on a man that is never going to marry her, never going to give her the kind of life she wants and really needs. Even he thinks she deserves better than that. But everytime he tries to break it off with her, she refuses to accept it. How can I help her let go?