Should we pay for a friend's wedding expenses, after they accepted ...

We offered to pay over half (they are responsible for plane tickets only, and some meals) of my best friend's expenses to be in our wedding, his wife and him are now saying they can not go due to money trouble. .... The next thing everyone knew, after they had paid for their plane tickets, they had rented a car and a hotel!! WTF!?!?!?! You HAVE to draw the line.. Just tell them, "I'm sorry, but we are having a big expense with the wedding and we really can't afford to ...