Your poor friend. Dealing with my own break up at the moment, my heart really goes out to her.
However reading it all, I'd be inclined to agree with you. It can be so tempting to just tell her what she wants to hear so I can understand why you'd want to keep it positive for her own sanity. But, speaking as someone who was given a butt-load of false hope in the last 5 weeks since my own break up, it's better to be honest.
Otherwise, it might just prolong the agony. And she doesn't want to hang on for him forever.
It does sound like he's put a lot of thought into it...not many people would casually end a relationship of that length without giving it some thought.
Of course, she's going to believe what she needs to believe at the moment, but I definitely think you're right to tell her what you really feel about it rather than what she needs to hear. He might well come round, and for her sake I hope he does, but if he doesn't, at least with your help she'll be better prepared emotionally.
Go to Glasgow at least once in your life and have a roll and square sliced sausage and a cup of tea. When you feel the tea coursing over your spice-singed tongue, you'll know what I mean when I say:
'It's good to be alive!'"
- Billy Connolly