I am very sorry for your loss. Had Charlie been ill? Would you consider necropsy? That's a hard thing to authorize. The neighbour must be worried and upset she possibly did something wrong. In a similar situation, my Mum looking after someone else's dog, we found the pet seemed to just give up when the owner went away. Don't tell your parents that, no need for them to be thinking any fault accrues to them for going on a vacation. But my Mum actually took a dog she didn't own and had it PTS, it was in terrible, terrible shape that the owner could not see.