you can take on someone on a zero hour temporary contract to help out, i assume you are saving on the other persons wages, negotiate a return to full hours in say 6 months and have regular conversations about it, take on a zero hours person to cover and give them a 6 month temporary contract, then if your employee feels they can return to full time you can adjust the temp person's hours. Don't aggravate or alienate your employee at this stage it is too early in her rehabilition process.