I am no expert but you usually place then right on the sore area, everyone is differant and may not react the same, if he wants to try it i would play safe with a very light strength to start with and see how he goes, as you know very well unfortunately how the nerves can react as its part of the small nerves job is to sense vibration.
I placed my electrodes on the top of my feet, on a light slow pulse, i tried the soles but didn't feel anything and that was the same with my magnets they only worked on top of my feet, but those i left on all the time, except for bathing and they worked much better than the electrodes probably because they were on 24/7 and the electrodes were only for 3 or 4 hours at a time.
good luck
Brian