The best way to do this is to look at how Arthur Miller himself portrays Biff's reaction. Biff is horrified, angry, and extremely disappointed by his father's actions, and the disappointment (and dramatic irony) is exacerbated by Biff's purpose in seeing his father in the first place: he is convinced that his math teacher, "when he sees what kind of man" his father is, will be persuaded to change his grade, thus allowing him to play college football. Then Biff discovers his father's mistress, and tearfully rushes out of the room shouting "You fake! You phony little fake! You fake!” So a diary entry in which Biff describes this event would need to use words like "admiration" and "respect" to show how Biff felt about his father beforehand, and "disappointment" and "heartbreak" to show how his opinion changed. Biff was despondent because he no longer respected his father.
You should also, perhaps, include Biff's tears and his lack of concern for his future. Because he no longer respects his father, he no longer has the drive to be successful to impress him. So while tone would be important, discussing how Biff understands the event in terms of its consequences for his life might be even more so. The diary entry should reflect a great deal of soul-searching on Biff's part, and not with a positive outcome.