
C.Section and they left one in there

Obviously your poor bitch will need antibiotics and you are VERY lucky she's not very ill right now. Personally much as these vets need to be told, there's no way I'd set foot across their doorstep ever again.
I don't do sue, but in this case SUE THE B**tards. You might like to take this further, if in the UK contact the BVA or RCVS who have a complaints department. This is disgusting and puts me in mind of my tragedy, years ago now, when I took my bitch in for a Section which wasn't done until the morning by the locum. My regular vet was on holiday and the admitting vet the Senior Partner who I discovered afterwards 'didn't do surgery'. Result - 9 puppies 4 doa, 1 died shortly after coming home, with milk coming through his nose from the staff trying to bottle them. The four survivors included one who became Top Puppy in her breed that following year, with 1 CC before being exported (behind my back). A sister we kept took one RCC but hated the ring so I let her be a stay at home girl. Both boys took their Stud Book quali. These were the survivors. Heaven knows what the other 5 might have been. We did sue but how do you quantify what might have been, or the fact we dare not risk another litter (which I don't do after a Section in any case). Some vets need stringing up.
I hope your bitch recovers well.