A big guy like that needs more than a 500cc or 600cc.
A 750 or 800 would work. Not balls out fast but highway capable.
I know a guy 6’7″ over 400 lbs. who rode a Volusia 805 for years and it worked quite well.
He’s gone onto a bigger bike now, but that 805 did the trick.
Tell your’ buddy to look for a nice used bike to start with.