This is a simple one. Make other plans.

You say "I invited you on this trip as a way to celebrate graduating and I'm sorry if I failed in any way to make that intention clear. I invited you because we are close friends and I wanted to celebrate graduating with you. I'd planned on this trip with people I know at a time that works for my schedule. It sounds like you're envisioning something really different. Unfortunately I'm not going to be changing the date of my trip to accommodate someone I have not met. Nor am I comfortable with you bringing a stranger along. I'd suggest you take the trip with your friend in in June/ July. I won't be able to come, of course, given my schedule. I'll be planning a different trip for myself in August that will be open to close friends of mine as a celebration as was originally planned. If you decide you want to come on that trip too we can talk more about it, but I'll admit to some disappointment and hesitation given the way this was handled. I value your friendship and hope we can discuss this further. Please understand this isn't about excluding your friend, it is merely about me choosing to celebrate a huge accomplishment in my life in the way that feels best for me."