As long as the volunteer is covered by the employers liability insurance it shouldn't matter. whether it is a paid employee or a volunteer they should be covered by the shop's insurance, plus, if they have a disability or any physical problems that mean they may need assistance in the case of an emnergency evacuation, there needs to be a dedictaed risk assessment done for them. lots of other health and safety issues might apply too, but charity shops tend to be quite strong on that kind of forward planning.