Atherapist who sat with a patient with bulimia while the patient ate appropriate quantities of "forbidden" foods, and then stayed until the patient no longer had the urge to purge, would be practicing:

Exposure and response prevention

Exposure and response prevention (ERP or EX/RP) is a type of cognitive behavioral therapy where an individual is asked to expose himself/herself (E) to thoughts, objects, situations, and images that trigger a patient's fears, anxieties, or obsessive/compulsive actions.

The patient then consciously makes a choice to discontinue his/her usual responses to the said exposures and makes a commitment not to give in and engage in the compulsive behavior (Ritual Prevention - RP). This is initially done in the presence or guidance of a therapist. Eventually, the patient learns exercises to manage and cope in the absence of a therapist.