We do know Eliza II's approximate age, that she seldom saw Colonel Brandon (due to his commitments), that his demeanour is grave and dry, and that her virtue wasn't enough to stop her from roaming around Bath unchaperoned and getting seduced. I don't think it's much of a stretch to suppose that she could speak of Colonel Brandon flippantly and ungratefully.

She wouldn't have supplied the specific bits about the brown mare and the faultily-hung curricle, of course. They'd be Willoughby's own material... and perhaps not even true. I doubt that the Colonel would get into any discussion of the curricle, though he might perhaps have frowned at it.