As builders says, there is nothing specific.
If your friend was a scaffolder off 'sick' with a broken leg and turned up on crutches to have a pint with you, then their employer could have no problem with this, as the reason for being signed-off has nothing to do with fitness-to-drink, but the ability to do the work.
However, if your friend was, say, an office worker and signed-off with severe vomiting and diarrhoea, yet turned up in the pub and stayed drinking all night..... well, I still don't think their employer could go as far as dismissal, but your friend would certainly have blotted their copy-book......