It's a joke. "Fifteen big ones" means fifteen thousand dollars. Rachel owes the most money and the speaker is making a joke about it by pretending that it is much more than it really is. People make many feeble jokes of this kind. Friends tease each other by making feeble jokes, and show their friendship by laughing at them. Sometimes someone will say something like "and the bill comes to three hundred and sixteen dollars" when the bill is actually $3.16.

Rachel owes 15 dollars.

I tried to find it online and it says that "big one" = "1 thousand dollars" Here is the link: http://dictionary.reference.com/browse/big-one