Next, what does the purchase contract say? You and the buyer are bound by those terms. Any money that the seller put down as an earnest money deposit is at risk if she fails to follow through on the agreement. And your contract should have had very specific time frames for each step in the process to occur.
I'm not a lawyer, so I can't address the legal recourse you may have. But your primary recourse is to enforce the conditions of the contract. Practically, you can't force her to buy the house. But--as noted above--you probably can retain any earnest money deposit and terminate the transaction.
Again, though, consult with your Realtor.
P.S. If she sold stocks to come up with the $15,000, then she shouldn't have much trouble. The stock market's been doing great this year--some sectors up 40% or more.