I'm not operating crew but something in this just doesn't hang together. As others have pointed out, the point at which the Captain became aware of the situation is important. But if that quote to the Sun ("The pilot saw for himself that the crew member was in no fit state to work and engage with passengers") is anything close to the truth, surely the Captain had other responsibilities and actions to take. It does rather sound like the issue became apparent on the return leg - any earlier and the Captain may have questions to answer. And in this day and age, the Captain had little option.
On a more personal level, I hope the crewmember gets help if she wants it, and that it's a wake-up call to anyone heading in the same direction.