If a car is sold on monthly payments, who is entitled to the insurance proceeds if there is an accident?

It would be legal to do it either way--either transfer the title to buyer immediately, subject to financing and (likely) repossession in the event that payment is not made (this is how most financed car sales through a dealership are done); or to have title transfer only on full payment. From what you describe, with title still in your name, it appears this transaction involoved the second option. In that case, you were still the owner and would seem to be entitled to the insurance payments; however, you would have to refund her all payments she made to date, since the conctract of sale has now been made impossible (you can't provide the car to her, since it was totaled), and when a contract is made impossible, it is voided and each party gets back what it had provided. In this case, she gets her payments back, and you get the car (or in this case, full entitlement to the insurance proceeds). You can't keep the insurance and her payments both.