Who tells Claudio that Hero is being unfaithful?
Don John, Don Pedro's illegitimate brother, in Act 3 scene 2. It is a lie, and Don John seems to be motivated in part by jealousy of Claudio and of Don Pedro. He is of a melancholy disposition, as Hero says in 2.1, and seems to get pleasure only in denying it to others.
Don John tells Claudio that Hero is disloyal.
In Act 3, Scene 2, Don John, the illegitimate brother of Don Pedro, nicknamed the "Bastard", told Claudio, a young soldier who had fallen head over heels for Hero, that Hero is being unfaithful towards him. It was a fabrication told by Don John but Claudio was gullible to listen to him and thought it was true and he would disgraced her publicly during the wedding's ceremony the next day if that claim was true.