What does her father think? Is it only you that cares?
I can actually see that it is acceptable if the individuals are happy.
My XH earns more than me, but I earn a good wage. I don't need child support, and though I'm 70/30, he also needs to provide a home for her. I pay for all her activities (a lot!). School trips - primary so not much. The only thing he pays is a £100 share per month of after school club.
I am entitled to about £400 a month but I have never asked for it - I just don't consider it necessary (much as I flipping hate her father!) I am quite happy to pay for my own child - your husband might be too.
In this circumstance too, it's not a simple choice of where she lives. It was 50/50. Her mother may have decided that it was an amazing opportunity for her daughter to grow up in this country. It is impossible for me to imagine making that choice myself, but a father is equal to a mother (at least in theory - plenty of both are arseholes!) so it was a valid choice.
Now if I let my XH have my daughter full time because it was in HER interests not in MINE... Well tbh, I wouldn't want to also be paying for the privilege of not even having my own daughter! It sounds like she hasn't dumped the girl on him and walked away - she has given up a lot. No way would I want to me paying my husband to have her when I actually wanted her anyway! She has potentially given up something she really wanted, for the sake of her child. To be asked to pay for that too? Especially if the father is able / willing to pay. I can absolutely see that individuals might decide this is perfectly fair - including the flights home.
It's not a simple straightforward choice for the mother not to do 50% - the father's move made it impossible.
As I say, I speak as someone who has turned down £400 a month of easy money, so I think my money is where my mouth is.
If the parents are happy - then drop it.