I disagree.
You can love someone very deeply, and then something happens that is like a bucket of ice being tipped over your head and suddenly, you're seeing that familiar face as if for the first time and it's not at all what you thought it was. Often referred to on Mumsnet as 'getting the ick'.
She saw her future with OP's son, but that conversation a few months back showed her that he didn't really see his future with her; she saw that he didn't think of her as Miss Right, he saw her as Miss Right-Now.
Bucket of ice, slap in the face, woke up and smelled the coffee, shock to the system, wake-up call, scales falling from her eyes, getting the ick - call it what you will, it happened, and as a result of that conversation.
Or, more likely in my opinion, it's been building slowly, her love dying the death of a thousand cuts served up by his thoughtless taking her for granted; and that last-hope conversation crystallised that she'd been kidding herself for years and he wasn't committed to her at all, so why flog a dead horse?
(Apologies for all the cliches, but sadly I see what has happened to this couple as just that.)