i mean i think shes a bitch in the present, not necessarily in the past or at whole fault for the divorce/breakup,
what happened 14 years (at least) in the past is still mostly unknown, but in this interaction yes shes the bitch, shes remarried with a 14 year old her own, seemingly well adjusted and happy. But the mere sight of him turns that right around on a dime, almost like a mask coming off, while our MC at first was still clumsy but gradually hardened over the course of it as he had redress himself in his old mentality.
But yea, 14 years at least married, knowning fMC probably likes him but goes out of her way to throw his insecurities in his face to undermine their chances, shes neither moved on nor willing too.
So yea, she might not have been the bitch or cause of the break, but in this situation she is indeed the bitch at this time