Aggressive dog

What you can do depends on how amenable your neighbour is because it's really them who should be taking action.

If this is a sudden change in C's behaviour with no apparent cause, he should really have a full vet check, though that's probably not possible. Sometimes, it's the 'victim' that has developed a health issue, so look out for any changes in R's health too.

Your neighbour needs to keep C far away enough from R that he doesn't feel the need to react - she can then change his association from seeing R and wanting to attack to 'See R - owner gives me a great treat, YAY!!' This means that he'll have to be on lead whenever they are in the garden at the same time. Muzzles may prevent injury, but being attacked will still be very damaging for her mental health and probably turn her into a nervous dog who may well become aggressive in turn.

But the easiest thing to do might be to have a schedule so you simply avoid any meeting between the two of them - or her putting C on lead whenever they come into the garden and only letting him off when she can see R isn't there (and you checking C isn't there before letting her out).

 

Bear in mind that the treat isn't a reward for not being aggressive, it's something that happens whenever Rosie appears regardless of what Charlie does. You don't want him thinking 'Must be good must be good must be good' (because then he's having to suppress his aggression and one day he might not manage to suppress it), but be calm, happy and chilled about seeing Rosie - which is why in an ideal world he should always be at a distance where he is calm in the first place.