My boyfriend left for another country to vacation with a good female friend of his, should I be worried?

He has remained in contact with this friend of his whom he met in college, they have a lot in common, and the girl is currently studying/working in India. My bf has to go to Taiwan for a business meeting in three weeks, but without consulting with me first, he decided to fly into Thailand and vacation there with this girl for two weeks before going to Taiwan. His version of of the story was that his friend invited him to go to Thailand with her since she is already in India, which is very close to Thailand (a four hour flight to be exact). What bothers me is that neither of them has any work related business for traveling to Thailand, but they were still willing to spend five six hundred something dollars out of their own pockets to fly out of their respective countries just to meet up and vacation together. He said they will be sharing the same hotel, too (and most likely the same room since that will be cheaper for both, but I didn't ask about that part). When he told me of his plans with this girl, he had already been planning it for weeks, and had already made hotel reservations and bought the plane tickets. Although he tells me they are just friends, and that he has every right to have friends of the opposite sex, I still can't help but question his motives for wanting to vacation with a girl for two weeks without ever suggesting of inviting me along, especially considering the fact that he dated this girl back in college (she rejected him back then). Should I be upset with what he did and is this enough ground to break up with him?