We had rats in our garden and so did a chap down the road...both sets of rats came up from the sewer both of which had become partially blocked by the raubbish put down the lavatory and sink plug holes....once where they had come from was exposed then pest control put poison down and himself blocked off the sewers overflow pipe....as did a neighbour some 10 houses away.... pipes join up in groups of 3 before flowing to the main sewer....it therefore may not be your neighbours fault that they are there....if she can't look after her garden I amsure that kind neighbours can help her