I don't think it a question on the volume of stuff they want to bring along, it more a question of weight.
I find it hard/cannot pack things down small due to being single handed.
So whereas my gear on a trip takes up a lot of volume, it not that heavy.
The down side is that spare space does tend to get filled if your not carefull.
The other thing I've worked out is that I slow down ~1 mph per 10 kg of gear lugged in the trailer.
Without the trailer I'll average ~12 mph on the trike.
With the added resistance/weight of the trailer, this slows down to ~10 mph.
Add the weight of my camping gear and on a long trip I'll average ~8.5 mph.
Now add a hilly route when fully laden and that can drop to a low as 6 mph for the day.
This was when my ultra low gear of ~12.5" came in very usefull, winching up hills at ~3 mph
So its then a question of how far you want to go in a day.
As I was only aiming for ~40 miles per day, going at 6 mph was ok as it gave me a full day's ride.
So what type of dayly distance/speed where you aiming at ??
And do you think he would be fit enough to keep it up for the full tour ??
Remember one advantage of a trike is the ability to go very slow with ease.
Luck ............
