I have a fridge/freezer with the freezer compartment at the top.
Many years ago I noticed a wet patch at the top of the fridge/freezer – initially I thought it might be water dripping from the ceiling – having eliminated that I realised that it was condensation.
Although I have conducted no further investigation (the wet patch remains) it is also certain that the thermal insulation between the internal freezer compartment and top surface has been compromised. Either the insulation was never very good when manufactured, or ice build up due to not being defrosted as required, breaking through the internal thermal insulation.
Either way, I suspect that your sister’s fridge/freezer not to be an expensive item, the condensation may be made worse if for some reason she has high humidity within the room that the fridge/freezer is situated.