Surely a muzzle doesn't really tackle the root of the problem-and also doesn't it prevent access to water, and food/toys-thus exacerbating the problem.
I am surprised your daughter's neighbours haven't responded to a polite chat-most people prefer that to having authorities involved straight away...
redhead23's idea re. CAB and involving other locals is really good. I think you can phone the RSPCA for advice rather than to notify them of the problem. If the dog is being left for too long, and it is this unhappy, then it's definitely not fair. I understand that people have to work but allowances should be made.
Could your daughter's neighours not ask a friend to pop by and spend time with their dog? Is there any way your daughter could help out?
Good luck, Lisa x