The passé composé is also used when an action has a clear beginning. It's hard to make this distinction clear with "to be disappointed," so I'm going to use "to be sick" to show the difference:
J'étais malade = I was sick (for an unspecified period of time)
J'été malade = I got sick (e.g., after eating some bad seafood)
So in the case of déçu, you need the imperfect when their disappointment is ongoing without a beginning *or* an ending, but the passé composé when they suddenly "got" disappointed because he cancelled on them.
Does that help?
Sign in to submit your answer
Don't have an account yet? Join today
Test your French to the CEFR standard