Go over to his/her house/apartment/dorm to help him/her study.
Call him/her to help them study.
Tell him/her you are sorry, but you have to study yourself.
Ignore what he/she said.
Say,"Well, I am sorry, but I don't believe that. I trust him/her."
Say,"He/She tells me everything."
Say,"Really? He/she is a liar then."
Say,"I HATE him/her now."