Bonsai trees are planted in very small containers with very little soil, so they can dry out and die very quickly. My neighbour has some very old and valuable ones that he keeps on his patio and when he goes away I have to water them daily, and sometime even morning and evening in summer, else they can dry out completely, even in half a day. Your friend's comment about it not mattering where it stood however I think was wrong. If you've moved it somewhere with significantly less light, or somewhere warmer/colder, or even into a draught then that can really affect them very quickly. It's better that a carer goes to a bonsai rather than a bonsai coming to a carer.