Ajapale is making the legitimate point that work performance deteriorates over a given continuous working period. In some occupations (I'm sure not those of the OPs GF) this has serious consequences for the worker and others. I would hope any such professions have specialised guidelines and processes on this matter, though who knows.
We can't overly control people or workers. And some of them will perform poorly for reasons we can't control (I had alcoholic and mentally ill teachers during secondary school, and physically violent ones in primary school). There should however be legislation in place to try and avoid people from working too many hours, though I imagine 48 hours in the current climate might be restrictive. I would assume that figure wasn't plucked out of the air and is based off some research that indicates undesirable consequences if you regularly cross the 48 hour mark. I have no idea how this compares with our European neighbours, or what the latest research has to say about working hours.
That said, if what my HR manager told me recently is correct, the 48 hour directive only relates to paid employment. Quite why you should be free to run yourself into the ground for no pay but not for pay isn't immediately clear, I suppose it's what we call a point of principle.
