The incident took place at Cape May, N.J. on July 21 night, and Hathaway, 25, was seen taking turns behind the microphone at a karaoke session.
The actress, who felt the need to sing Journey, had arrived at the Martini Beach restaurant on the Jersey shore with friends and family.
"She was the life and soul of the party," People magazine quoted karaoke host Terry O''Brien as saying.
Even with the split, she was all smiles and was even one of the first singers of the evening crooning Journey''s "Don''t Stop Believin''"
Hathaway danced around the tables during other songs before retaking the microphone for K.T. Tunstall''s "Suddenly I See" (a song featured in her movie The Devil Wears Prada.
"Don''t put this on YouTube," she told O''Brien.