Hello Mike
Mice can get through some remarkably small holes (size of a pencil) so the obvious first thing to do is to check that all the ventilation holes have their mesh fully in place, and for any other holes such as poorly fitting main door and windows, fridge vents/covers.
It may also be worth having the gas heater fully inspected for correct fitting, if the floor plate is not properly seated there may be a mouse sized gap - I must stress that is is not dangerous from the heaters point of view.
I agree that setting traps or poison has a rather unpleasant side effect, so finding some way of discouraging the rodents would be preferred.
Changing storage location may be one rather drastic solution.
I do not know if your friend has access to a mains supply, but if he did then you can get plug in electronic devices that claim to make the environment unpleasant to rodents and thus they prefer to go else where. Whilst I cannot claim scientific evidence for their efficacy, following a=an infestation of mice in my attic, I installed a couple and since doing so I have not had any further evidence of mice.
There may be a company that does a 12V version, and by using a solar panel, the battery may be topped up enough to keep it running.
I am also told that moth balls may act as a deterrent.