Don John takes Claudio (brown hair) and Don Pedro (short, black hair) aside to confess something to them. Claudio and Don Pedro are suspicious and are curious to know what it's about.
When he them that Hero has been seeing another man, Don Pedro is shocked and Claudio is gutted. But they weren't completely certain that they could believe this without proof. So, Don John tells them that if they look through Hero's bedroom window at midnight, they could see the disloyalty before their eyes.
Then, Don Pedro and Claudio vowed to Don John that if they really do see Hero cheating, they will publicly embarrass her about it in the wedding.
That night, Borachio, a follower of Don John, snuck into Hero's room and made Margaret, one of Hero's gentlewomen, help him with this endeavor. They acted as if they were Hero and another man together - Borachio calling Margaret 'Hero' time to time to make Don Pedro and Claudio standing outside more convinced.
After processing what they had just seen, Don Pedro and Claudio were more convinced than ever. Claudio was extremely depressed that the love of his life had been disloyal, but they both decided that they had to stay with their vow and were going to go ahead and publicly embarrass Hero the next day at the wedding.
At the wedding, Claudio publicly embarrassed Hero, as he vowed, and Hero was extremely shocked to have been accused of doing such a thing. Don John's plan had worked!