Destination weddings that people expect others to attend are obnoxious, imo.
Tell her that's lovely she's having the wedding she wants, you're sorry you won't be able to attend. If he can go alone or you two can go as a couple at a minimal cost and want to, agree that'd be an option. Otherwise, they chose the destination wedding, they don't get to choose who attends.
A family member of mine wanted a destination wedding where they'd met - so they had that there, with a literal handful of very immediate family members they paid for. They sent everyone else a pre-wedding thing explaining that they wanted to do this there for this reason, and would not ask people to go on a trip, so they were just doing it alone and when they got back, were hosting a reception to which friends and family would be invited to celebrate with them.
I thought that was a considerate and nice solution. This assuming people will shell out thousands to attend their wedding? That's their problem, not yours.