I think it's not wrong for you not to inform him as u are not obligated to do so. But it's not very nice to purposely make use of his time to think so long :P though it's not your fault also since he forgot to press the clock haha. I remembered in the past someone (not to mention the name to protect this person's interest) was playing against Shen long chun. He was going to lose but shenlongchun forgot to press the clock in the end 30 mins later Shen's clock dropped and lost.

棋品 Is very hard to judge and it varies between individuals, Most importantly feel guilt conscious whatever people say is not important if you think you did nothing wrong.