Nothing like a bit of ambition! I think it might be fun to sit down and go through it with him as a lesson in planning and logistics if nothing else.
What route is he getting 200 miles from? If it's as the crow flies or via motorways the actually route he'd need to walk may end up being a very different distance.
What terrain would it be? Ten miles on the flat is very different from ten miles over the pennies.
It's doable, although I think ten miles a day would be pushing it for twenty days so I think I'd want to make it more like four weeks/a month to allow for some days off, shorter walking days etc.
I would also want to vary the length of the days - some days 12-16 miles, some days 4-5 depending on terrain etc.
Then take him for a ten mile walk and see what he thinks afterwards.
Logistics - in an ideal world, it'd be helpful to have someone with a car nearby to drive you to accomodation etc if it's out of the way and in case of emergency/terrible weather etc resupplying if necessary etc.
200 miles is a bloody long way - to put it in to perspective it's like walking almost half the distance from London to Edinburgh!
I'd be inclined to encourage him to start a little smaller maybe - fifty miles away or something, which should be doable over five days.