Does he mean 10 consecutive days? Why 10? If he hasn’t done longer fasts before maybe he could consider starting with 24 hours and keep increasing by 12 or 24? This gives him the opportunity to adjust to various factors within himself and in his home environment before committing to 5-7 days.
And I would suggest getting his doctor onboard first for baseline blood work, especially if any of those longer fasting days are going to occur while he’s on the road.