Pretty soon she will be in school and won't be able to visit her dad every other month.
I know it doesn't sound fair, but, throwing her in another world every two months for a week is probably the main reason she doesn't like to visit. It has nothing to do with her dad. It has everything to do with being uprooted, away from her friends, her toys, everything she knows and loves.
What I would do is change the number and amount of times that she visits. I would set it up so that he could have his daughter every summer for one month. This means that she would not be moved around continuously which is really not good as she resents her dad for it, and spending one month at a stretch would give the dad a better chance at connecting with his child. In addition, this is when he should take his vacation time, during that summer month when his daughter visits so that he has ample time to connect with her on a 24 hour basis for at least two weeks or better yet, 4 weeks.
It is always such a disappointment to me when the parents don't make the effort to make the child's life easier with simple explanations. For example "I know you don't want to see your daddy because he lives so far away, but, he is your daddy and he loves you and wants to get to know you". When I got a divorce, I could not imagine what it would have been like if all the parental responsibilities were on my shoulders. I insist that my ex be involved with my kids and the more the better.