How to tell my friend that i caught her in a lie but not rudely? [details inside.]?

i asked my friend to be my model for a photography project in school like 3 days, and so she said yes. then the night before she called me and said she was really sorry but she completely forgot she had plans with another on of her friend so i was just like okay. then in school i found out she lied and actually made the plans 5 minutes before she called me.. i know its not that big of a deal but i feel like i should still confront her about it so what should i say with out being rude?