With your parents situation it will probably be easier to drain and refill. You are not going to be able to complete the slam process long distance. Next time you go up there, drain about 75% of the pool and refill it if you can do so within 48 hours. Add enough CYA to reach 20ppm and raise the FC to shock level, check it in an hour, and raise it back to shock level if needed. You may also want to install some level of chlorine automation either with an SWG, Stenner pump, or even an inline puck feeder. Just keep in mind if you go with the puck feeder that you will need to keep an eye on the CYA level and be prepared to drain if it gets too high.