What do I do to treat this wound on this dogs nose, and what do you reckon is the cause of this injury?

Call a local veterinarian, no one here can give you good advice

Call a vet, and get the dog in.

I agree that the best course of action is to call vet(s), with hope that somewhere, someone who is qualified can give you their opinion and some tips.

Since it’s not your dog another option is to call some rescue groups or even shelters.