Neg G's  

Just because it rolled doesn't mean it had negative G's (it depends on how the roll was done).

ANY helicopter can roll...

https://www.youtube.com/watch?v=MQ7pVjQ5Y5g Sea Stallion

https://www.youtube.com/watch?v=q52KxdjXvJM Apache

https://www.youtube.com/watch?v=USmOqlj9Hag Blackhawk (semi-roll)

https://www.youtube.com/watch?v=QijetGUlJ4A Lynx

I've known a Bell 206 pilot who did a loop, and various old UH-1 and AH-1 guys who have done it (the Cobra guys did them from altitude in an aggressive right/left dive to inverted, so I guess that wasn't a full roll from level all the way up and around but seems like a roll to me).  So even tetering rotorheads can do it if you maintain positive thrust.

There was a video posted on this forum once of an R44 doing it.  It was grainy old phone video but it was very distinctly a roll.  It looked like he was trying to do a return to target and rolled it over accidentally.

Obviously, any articulated system can do it with ease as long as you don't reduce collective while inverted, which would take off the tailboom.