Hi Octavius, would ACAS be of any use for info, this is their no: 08457 47 47 47.
I always was under the imression docs dont give sick notes until you have been off for 5 days. Of course, your wife could now go and ask to be signed of until she feels good again and that would really make her boss tetchy, and she could do nothing about it. And after all, she did ask for a certificate.
Sorry I cant help on the legalities of contracts. Might be worth you dogging out your wifes contract and see how that reads on sickness.
Do wish you all the very best, and please make your wife some ginger tea or ginger cordial. It should help with the sickness :-) xx