Just to update; sadly the lump is aggressive mammary cancer and is pretty far advanced. She's obviously not insured and her owner doesn't have the money to treat her but even if he did, because it's so far advanced now any treatment might only prolong her life slightly and the vet wouldn't recommend it anyway. He said she could have a few months still and currently she's behaving like a well dog and seems very happy so my neighbour is going to just see what she can do and play it by ear.. she's going to speak to the dogs owner and suggest the dog stays with her mostly for respite type care and I've said I'll try and help out when I can if she needs to go out as Tarka has really bad SA which clearly is not something that can be addressed at this stage.
Very sad story, none of us know if it would have been treatable if her owner had done something about it when it first appeared, but there is no point dwelling on that now.