I'm puzzled though.
If the company (whichever it is) has a duty of care, what care do they have to take.
Well they have to do a risk assessment.
So the risk is that there will be drunk and possibly violent people entering the store.
The solution to that risk is - Employ a security guard to deal with them.

Maybe the company that employed him should have made sure he was trained to deal with drunks and knife attacks.