If he's been saying this is the one, he's probably been coming on a bit too strong, and it may have put her off. Or she doesn't fancy him.
I'd just tell him to Man-up, they must be pretty friendly with each other for it to get to this point, ideally you would want to try and get back to that pre-going out relationship, write it off as a silly idea they both had. Laugh it off and play it cool, if she should then again show an interest, PLAY HARD TO GET, this never fails and just see how it plays out.