Hi Kisuk,
I had a Skype interview but I was very clearly told that it's a formal interview and I was one of the candidates. Is it the same in the email she received?
A friend of mine who was admitted last year also received a Skype interview first, but then he asked whether he could come to the interview weekend, and people in our department agreed and offered this opportunity.
I think our department assumes that it's more convenient for international students to be interviewed on Skype, so that they are first offered Skype interviews. But it's also flexible if international students would like to come. Just my guess.
It might also be a good idea for her to email back and ask whether this is a formal interview.