I deffo agree with nova on not telling her you were going to flake, it sounds like when you have a conversation with someone and you agree with them even if you don't. For example if she says I hate cheese so you say you do too even though you love it.

I'd have said something like, that's ok i'm free friday and tuesday, and if they can't make then I try not to cancel any of my plans to meet up with a girl. Your time is valuable!