Hot or cold air contains water vapour. When warm air hits a cold surface then the moisture in the air just turns to water which is how condensation forms.
There has to be a barrier of insulation to stop the warm air touching a cold surface.
You have to decide what kind of barrier is going to fit the customers budget.
Probably the cheapest would be dabbing insulated boards to a wall. But it wont be the most effective. It may be the cheapest.
The most effective would be to insulate and stud out. This would be the most expensive also.
Just give them the options and costs and let them worry about it and move on until they make a decision.