Nige - if he retains positive G throughout, the helicopter doesn't know whether it is inverted or not and there is no difference in the stress on the aircraft at that point than there is in a 'normal' wingover.
As someone pointed out earlier, the recovery with high speed, if you let the nose drop on exit too much, is probably the most stressful - but no more than flying at VNE when heavy and encountering some chop or turbulence. Would you sack a pilot for having that experience and 'overstressing' your aircraft?