fuimos, in this sentence, is not a conjugation of ser, so it does not belong in this grammar exercise.
This is one of the tricky things about Spanish; fuimos is not only one of the forms of ser, it is also the past tense of the verb “ir”, to go. In this sentence, it is this verb. It’s an example of a homonym.
I think the translation would be better as, ‘My friend and I went to see a movie yesterday’.