I hope your friend is ok. I have a blue ox and I have forgotten to take the tension off one time. Luckily I did not get injured but it I do not forget anymore. Give your friend this piece of advice, do not have the handle up in the air when removing the chains. Try to have it parallel to the frame so if there is to much tension his head is not in the way. Hands may still get hurt. It's better than facial/head injuries.
The process others have posted are correct. Use the tongue jack to remove the tension. If you start driving and the front of the TV feels like it is wandering or there is too much bouncing pull over and re-adjust one link at a time.
2012 Chevy Silverado Crew Cab Short bed 5.3L v8
2014 Prime Time Manufacturing Avenger ATI 27BBS