Bonnie Flat spot at 60mph help appreciated

Technically you already have an idea to the answer to this as you stated that when you apply some choke it cures the flat spot.

Has the Bonnie been modified in anyway? Filter? Snorkel? Airbox? Exhausts?

Basically, at around 60mph, i'm guessing he's at circa 2,500 rpm the engine is running lean. Incorrect mixture, either too much air or not enough fuel to make the perfect bang! Hence, adding some choke temporarily cures the problem.
I would start with checking the airbox rubbers, vac caps on the carbs etc for air leaks (cheapest checks are the visual ones). Then i would strip & thoroughly clean the carbs & inspect those diaphragms again. Failing that i would look into the adjusting the mixtures or the possibility of re-jetting the carbs, shimming the needles or it could even be a worn needle. Some go down the route of swapping the standard Bonnie needles out for a Thruxton needles.

I had the same symptoms as your friend on my 865 America after modifying the airbox considerably, aftermarket filter, A.I removal & open exhausts. I changed my main jets from 120 to 138 but left my pilots stock at 42 & didn't need to shim the needles.

 

Having had great success remapping my 2009 Bonnie at a proper Dyno spot in Malton North Yorks, I took my carbed Harley. Test was £25 to find what needed doing if anything (was running lean and popping, needle in wrong groove and rejetting etc sorted. Different bike now). Total cost £83 and well worth it.

Always did it myself up to this year but it can be time consuming and yer need right jets , Harvs comments spot on, but if yer pals not too technically minded etc this is an option.Good luck