But that is a re-roofing job.
I assume that if your friend has been regularly storing things in the roof that this condensation is a new thing.
Check to see if the eaves ventilation has been reduced by fascia and soffit boards being clad with upvc. If it has been done recently get the company to come back to put more ventilation into the soffit boards and make sure the roof space has ventilation sleeves to ensure that the air can get up into the roof space from the eaves and blow through.
As has been said before check that the insulation hasn’t been taken down into the eaves to block the airflow within the roof. You can get plastic sleeves which fit between the rafters over the top of the insulation to maintain an air gap above the insulation and this means the insulation can cover the ceiling right out to the walls so that you don’t get cold spots at the in the corners of the rooms which can lead to condensation and mould growth around the edges of the bedroom and bathroom ceilings.
You need plenty of ventilation up above the roof space insulation if the insulation has been upgraded, from that which previously existed when the house was built, because the roof space would have been relatively warm with the heat escaping from below, when the house was built. With upgraded roof space insulation the source of heat from the living accommodation has been cut off and the roof space is now much cooler hence the condensation.
PM me if you need further advice.