How bad is the error? Is he saying utter rubbish or just a bit rubbish? A minor error I may let slide, but a really major one I would probably ask to talk to him and ask for his sources, as you read one that disagreed so would like to read more on the subject: you are studying this so it is clear you are already interested in the subject and extra reading can't be a bad thing. If you put it as YOUR mistake then it may come out better. Good luck!