"It is a treatment that is inserted by the right eye."
Why did the right eye need the treatment? Was it watering, inflammed, red, have an injury/ulcer etc? Their are different eye medications to be used depending on the problem. This particular one (vetropolycin hc) is generally used to treat bacterial infections of the eye that do not have ulcers.
Guinea Lynx :: URI
After looking at this I can see that if he had the crusty eyes the vet probably would prescribe the vetropolycin hc.
Your vet may need to do a culture of the bacteria to see what antibiotics it is resistant to. I do think you will want to find a different vet. It sounds as if the one you went to before is not very cavy savy and is not open to learning something new. You will want to find a new vet right away (immediately) as uri's can bring a guinea pig down very quickly.