Suggest Treatment For Acne On Back

I used a high frequency machine on my brother-in-law to get rid of one of his pimples on his back. And the next day it got super irritated and had a black dot in the center. I extracted it and got some puss out and then all that was left was a hole that was super red around it. Then the next day it came back? what could this be and how can i get it to go away?