Eavesdropper usually has a negative implication, suggesting disrespect for privacy. Being called an eavesdropper might offend him because it implies wrongdoing.
Pro Tip 2/3
Secretive Action
Eavesdropping is covert; the people being listened to do not know they're being overheard. The eavesdropper hid behind the curtain, listening to the confidential meeting.
Pro Tip 3/3
Accidental Context
Sometimes eavesdropping is accidental, not intentional. She became an accidental eavesdropper when she overheard her parents talking about moving.