It always amazes me how people ask total strangers about what bike to buy and they are spending 2k+ ........
If I was spending that kind of money I am pretty sure that I would be road testing bikes that were in consideration and making my own judgement
He can and will try all the bikes listed and have to make a judgement at some point, but if someone just said "actually that model/brand is a bit pants because" it would potentially save time. I looked at Cube for him as I was passing a dealer, the top tube paint/vinyl was blistered on six bikes(!) so that's them off the list, so if anyone chimed in with "double check the welds on x brand" we'd maybe avoid them, or check carefully.
As has been mentioned no-one has all these bikes to ride them side by side, so then it is down to a remembered first impression of the first bike when trying the second/third etc. And a simple hint of "x will feel really very twitchy compared to y, best to try y first" would help neutralise some of the geometry change 'shock' (longer chainstay etc).
Edited for better wording re geometry.