What type of converter do they have? How long are these periods of non-use? How conscientious are your friends about checking the battery electrolyte levels and topping up if needed? Is this during the summer or winter?
In general, keeping it plugged in would probably be better, but there are very many variables. Other good options include setting up a timer to power the converter for a short spell each day (say maybe an hour) or connecting a decent little maintenance charger and not using the converter while stored.
