It's not unreasonable to extrapolate from "got it from a friend-of-a-friend" to "quite a lot of people will have this".
Yes, one should notify the professor; but I think it's acceptable to do this a little less publicly (e.g. by writing it on the test or sending an e-mail), if one so desires.
Yes, one should notify the professor; but I think it's acceptable to do this a little less publicly (e.g. by writing it on the test or sending an e-mail), if one so desires.