Even harder to believe she was allowed to operate the return sector.
A possible response, even a likely one, is that minimum crew numbers and the Captain's desire to get home, sorry, avoid inconveniencing other passengers by delay/cancellation, were factors in his decision. Perhaps he discussed in with the company's Operations Centre (or whatever), and their main concern to avoid extra costs of delay, sorry, avoid inconveniencing passengers, drove the decision. Who knows?
I bet there's nothing in the Rule Book to say that you can't count all on-duty cabin staff towards the minimum if they are all present and correct, well, present at least, even if you suspect that one may need at some point in the flight to don a strait-jacket.
Afterthought: By the way, how come the same crew was rostered for the return sector? Quite a long duty, I would have thought. Or is there a minimum rest layover, and if so, couldn't the problem have been resolved during it?