zannussi frost free fridge freezer ER80341.

Hi all,
Hope someone out there can help with this one. Was out last week to this f/f and the evaporator in the freezer was solid with ice ( Her complaint was that it wasnt cooling) Took me about 20 mins to defrost with a hairdryer. This is a built in unit and i was under pressure to be somewhere else so I told her it most probarbly needed a defrost timer but that it would be fine untill I could get back the following week to fit it. 4 days later she called me back saying that it still wasnt cooling.I asked her to check for a build up of ice but she says theres none. The unit was cooling when i left it, with a nice film of frost on the evaporator and all fans working as they should. She tells me that if she leaves milk in the freezer section that it cools suffiecently but doesnt freeze. Any help appreciated.
Paddy