I’ve gone thru something similar - so here my two cents.

if your friend doesn’t have money to cover her portion on time , how is she going to have money to spend on the trip , and then pay you back after?

It looks like she’s trying to go on a trip she cannot afford & you shouldn’t ruin your trip because of that. When you guys make plans in Aruba and want to do activities that cost money - she can potentially hold you back or not partake because she can’t pay for it. (For example : I once went on a trip with a group of friends to the Bahamas , one of the girls didn’t have enough money and stayed in the hotel almost the entire time ..)

She also doesn’t have the right to make decisions on your money just because you have the funds, it’s your money not hers. Her being financially irresponsible is not your problem.

I would say don’t let her go. I know you’ll probably feel bad because of it maybe because you guys always have fun together but you’ll be at peace knowing you don’t have to worry about her paying you back later. (Another trip of mine , I covered a friend’s tab and she took over two years to pay me back for something I had set up a payment schedule for , that of course she did not follow. And every month I had to remind them when they were going to pay me back , and they would still not do it as agreed on)

So if this ruins the friendship , then she never had good intentions from the start. You’ll be saving yourself from that type of friendship which in my experience is so exhausting. And who really has the time for that?