It's his responsibility to bring it up, because if he thinks about it, he can relieve Heather from the obsession, and from having to make sure that the affair isn't forgotten, and that in itself begins to restore trust.
Il est de sa responsabilité d'en parler, parce que s'il y pense, il peut libérer Heather de l'obsession, et s'assurer que la liaison n'est pas oubliée, ce qui, en sois, commence à restaurer la confiance.