How do I ask a co-worker for money that she owes me when she is always crying broke but is going on vacation?

So I recently went on vacation with some co-workers and I of them went broke so I didn't want her to stay behind on any of the excursions so I paid everything for her. She said she would pay me back gradually when we got back. It' s been 3 months and she hasn't paid me back. I normally wouldn't care when she paid me back but now she is going on vacation again. Now my problem is that she is always crying broke but she is going on vacation again.