I would be harsh and say, "If you plan on having guests at your wedding, either you pay for all their airfares, accom, food etc or be considerate of how much they're spending." My SiL was exactly the same with her wedding. Some people just expect people to fork out their entire life savings to come to their wedding.
If she refuses to let up, book the worse hens night ever where it's dirt cheap and will cost no more than $20 AUD each. If she doesn't like it at least you can say you've never been to Thailand and someone said the place was AMAZING! How were you to know?