as you said, is a fantasy, is in the mind... and if the one with the fantasy (idea) doesn't communicate and acts alone the now idea, well that is not good; but if you have good communication, you should be confident in that link with the partner and you should be able yo talk it and look what the partner thinks... maybe will be "yeah! let's do it" ......
we don't know the complete story about the friend's breakup, we just know his POV... what if he was a bad bad boyfriend and she just kept waiting for him to improve... and 5 years was when she got tired and stopped caring... he never say anything about himself... soooo...