If your friend can't ride faster than 17 mph on the road on the flat the problem is probably more to do with having 2.25 " knobbly tyres than gearing.
I probably didn't make that clear, it's not that he can't ride faster but it's at that speed he feels he wants to have at least another gear available.
We've been going out regularly for over a year now so I can't see that his pedaling will change much from what it is at the moment and he doesn't seem slow ... I'd say he's about the same cadence as I am but I've another 2 / 3 rings on the cassette I can drop down to at that speed.
Not sure what difference it makes but it's a 29er rather than 27.5.
Cheers for everyone's help .... I don't think I want to be the one to tell him it's an expensive mistake though .... I may just suggest he takes it to a bike shop and see what they say and let them break the news rather than me!