The guy was rubbing his cat’s tummy which triggered tough play in the cat which went way too far. The cat grabbed the guy’s arm in a “death lock” with his front paws and teeth and scratched with the hind claws. It’ll happen in a flash and if you try and pull away you make it worse. At this stage it is a lose-lose situation.
What I’d do is to (1) keep the grabbed arm passive and (2) grab the cat by the scruff of the neck with the free hand. This should partly immobilise the cat which should allow you to release the trapped arm. Of course it’s far better to be proactive and avoid this sort of event. It’s about understanding your cat, being aware of his/her mood and behavioural traits and respecting the wild cat within.