I think the bristly hair tufts look very reminiscent of Catocala nupta, possibly a young instar? I may well be way off though. :)
It looks to have three pairs of prolegs, which would rule out Red Underwing. How about Light Emerald - a geometrid that has an 'extra' pair of prolegs (3 pairs rather than the usual 2). Grows to 4cm which is within the realms of a slightly exaggerated '5cm'.
Looks like you hit the gem right on the head there.I was indeed well off.
Thanks both - I'm sure the gentleman in question will be very grateful too.