Your grandmother's vet misrepresented the costs and should not now be charging this high an amount. I don't think she should pay anything until she has had avice from the CAB or the PDSA / RSPC. I would tell her vet that your grandmother was in fact lied to and hence is not willing to pay this amount. Get a list of and proof of the tests that were done and take it to another vet or the PDSA and ask them what it should have cost. If they say the bill is correct then you can still negotiate on the basis that she was lied to and would perhaps not have had the tests done if she had been told the truth. The CAB will be able to tell her if she can get help with the costs. Good luck!