At this point, you have two choices; you can either do the 'feel good' recommendation that was made and rescind the deal (costs you $$, with a SLIM chance of saving a 'friendship??), or you can do the practical thing of enforcing the contract. In either case, this is another example of why friends shouldn't loan money (or sell things) to each other!
If you decide to enforce the contract, do NOT take the car back. That could weaken what appears to be the strong position that you have. You have a contract, signed by the buyer, promising payments to be made. As long as you didn't make any warranty promises or have any 'money back' agreements, she has breached that promise and your 'friendship' has absolutely NOTHING to do with it. (My question... what value has she put on the 'friendship' in trying to MUTUALLY resolve this problem??? That will tell you the 'value' of this 'friendship'!)
Send her a certified letter RRR and (tactfully, but sternly) demand payment of the unpaid balance or you will be forced to take legal action. Give her a reasonable amount of time to respond (7 days??). If she doesn't, then take her to Small Claims court. If the facts (and documents) are as you presented them, you shouldn't have any problem getting a judgment. Then the only problem will be in collecting the judgment.