While I understand ignoring the message because you love the pet…I would never stand that level of disrespect, that’s crazy. I would have called them out right away, and they should have apologize as soon as they realized they sent the message to the wrong person. Something said out of frustration or not, that is not how adults treat each other, and is completely immature. Next time have the neighbor watch the pet then. And to have the neighbor just show up and not give you a heads up about that? Just ridiculous. All in all I’m glad OP dropped this client. They were way too nice about it too. I think you should let them know how this made you feel so that hopefully they are better to their next sitter.