If you want to cancel on her then do. She deserves better than someone who wants to cancel a date with her just because she sat with her friend instead of you. You don't know for certain whether or not she saw you, you're just making assumptions. Grow up, you're being possessive and immature, especially considering you aren't even together.
She kissed you and she confirmed the date, so she clearly likes you. Don't really see the issue to be honest.