I think you are quite justified. One of the perks of your profession is to be able to have off the summer.
Your friend (although I am sure it's unintentional) should not make you feel guilty that you have this luxury.
She needs to understand that this is your time off for you to decide what to do with, not for her to dictate how you should use it. And besides, just because you have days off does not mean you are not busy with your own personal stuff to do, no matter what that entails. It's not up to her to decide whether or not you are spending your time wisely or constructively or industriously.
