Respond with Empathy
...because we had been planning for months and I decided two weeks ago to invite the boy, and I chose him over her.
Whether intentional or not, you've made your priorities clear: the boy is more important to you than your friend. Your friend is seeing this and reacting. This isn't necessarily a good or bad thing.
To respond, I'd approach this from assuming two scenarios: First, assuming good faith from your friend, then not.
Assuming Good Faith
Imagine it from the other side: you talk for months with a friend in another town, getting excited about seeing them. Up until now, you've heard nothing but how much the other person is looking looking forward to seeing you. When you finally know when you can meet with them, they say they already made plans, and won't change them to hang out with you.
I know how I'd feel in that situation. My friend who had been talking for months about seeing me, perhaps even implicitly showing how important it was to meet me, won't change plans on the only week I can meet with them all summer. I'd feel lied to, maybe even betrayed. I might even get angry, and feel like lashing out at my friend, or feel like turning a cold shoulder to protect myself.
One option here is to acknowledge, empathize, and validate those feelings. In this situation, for reconciliation, I'd want empathy from the other person. I'd want to hear words like,
- I can totally see how you'd be angry/upset about this.
- I know it sucks to hope for plans and not be able to go through with them.
- I was looking forward to seeing you as well. I value you as a friend."
If this person is important to you, you might even consider offering a counter: visiting them instead, another week. if they can't leave, maybe you can.
In this case, you still react with empathy, but also assert yourself. (You might do this anyway.) Continuing from above, add "My plans with my boyfriend were set before yours, and I'm not changing them."
If there are repeated protests, return to acknowledging their feelings, empathizing with your friend's position, and asserting yourself.
An aside: Assuming Bad Faith
Your friend saying that the "only" week she can visit is the same week your boyfriend is visiting, and only after you tell her your boyfriend is visiting that week is suspicions to me. If that's the order, it might be a ploy to manipulate you to monopolize your time. If this is a one-off situation, you might pass this off.
You would still respond the same way, but assert your boundaries: you had plans already made, and you don't plan on changing them.
(In response to the comment:) I only bring this up to cover all the bases. You can not know what they are thinking, and assuming anything malicious without evidence (e.g., them outright saying it) or a consistent pattern of behavior, AND not addressing it with them, would be premature. If you've had a good relationship in the past, it's probably not the case. Communication is a fickle, complex process and it's easy to loose nuance when we're not around each other all the time...and even when we are. Unless there's a preponderance of evidence to the contrary, I'd continue to assume good faith. :-)