I think it is inappropriate for another girl to sleep at his house if you are not there. It's just too much temptation. But he should still respect your feelings. He should have talked to you about this before tickets were booked. But now it's done, don't overreact, or you will lose him. Have you told him that this REALLY bothers you how tickets were booked BEFORE you were notified? Your wording is important. If you didn't convey how much this really bothers you, he will think it's a minor thing. But you posted this on the internet, so it must be a major thing. So, did you actually convey that clearly to your bf?
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)