Beth is very suspicious that her boyfriend, ernesto, is cheating on her, and she constantly accuses him of being in love with his best friend, yukiko. however, beth is actually the one who is cheating! this may be interpreted as beth utilizing the defense mechanism of in her treatment of ernesto.

Projection

Explanation:she is cheating and she is probably feeling guilty about her own cheating and to deal with this she has to project these emotions to her boyfriend in order for her to deal with her own unacceptable behaviour , she has to shift the blame and put it on her boyfriend .

To make herself feel better she put the blame on him and convince herself that he is also cheating which makes her feel better about her own cheating ways.

i believe it is he has been given the drum as a joke.