I am a cancer survivor. I was actually given a terminal diagnosis with chronic pain. I was on massive amounts of pain meds for years, as I fought to live.
When the threat of death passed, there was an obvious need for me to be weaned from the pain meds. I wanted them out of my body so bad that I often made reductions in the dosage on my own. That is when I began to have similar experiences to what you are describing.
My doctor told me that it was caused by mild seizures. A seizure in the brain causing the pressure on my chest seemed to render him without an explanation; at least, I never got a response from him when I questioned him about it.
Strangely enough, I completed my education, as I was fighting for my life. I have very intense passion about the spiritual aspects of our physical existence. I know we are spiritual beings first. Things happen in our body, as a result of things that happen in our spirit, and vice versa.
I personally believe there was more to what I experienced, like this. However, I would suggest that your friend see a doctor to see if there could be a cause for him to be having mild seizures.