Don’t think of the journey as ‘how many miles’ they have to cover - but think more of ‘how many hours’ sat in the saddle. As Andrew says the saddle will play a big part, it would be very disappointing if you have to abort half way round because someone is in too much pain to carry on.
Lots of people don’t realize that many hours sat in the saddles can lead to so much discomfort, if you don’t prepare well - be prepared to be rubbed raw and if you are really lucky you might end up with skin ulcerations!!
I apologize for my next explanation but I couldn’t think of anything else!! Some guys also suffer badly from numb nuts! (for some this can lead to serious problems)
For me a saddle is a very personal choice, a certain brand may be great for one person but could be a nightmare for another. You also have the cut out saddles, I know a lot of cyclist who get numb nuts
(aren’t I a lucky girl!) and rate these very highly, but I also know a lot who have chucked them in the bin, so again it is all down to personal choice.
Decent shorts are also a must – but guessing that they already have the bikes and wanting to do such a journey, they already have this sort of kit.
All the more comfort and fun if they are fit, so I hope they train well, get some miles in the those legs.