Why are is sal disappointed in ms.Rushmore in ch.28 in walk two mins?

Sal is disappointed in Mrs. Cadaver (not Rushmore) in chapter 28 of "Walk Two Moons" because he feels she is moving on too quickly from his mother's death, as she starts going out on dates with Mr. Birkway. Sal interprets this as Mrs. Cadaver being disloyal to her mother and not mourning her enough.