Apart from physical damage, what sort of damage could someone do to your rig?
If the speakers are adequately matched to the amp then, presumably, the worst that could happen is that the volume is turned up to '11'. it might not sound great, but will it do any actual harm?
In my (limited) experience, the gear-sharing thing is usually to do with stage-space and minimising change-over times between bands rather than some players not having, or not being bothered to bring, their own gear.