Hi and welcome!
I have never come across the diagnosis of an overly acidic stomach in guinea pigs in the tens of thousands of piggies that have passed through this forum section (nor in other forums) and would definitely very strongly advise against using human homeophathic or home treatment medications in guinea pigs. Many human medications have turned out to have detrimental effects on rodents with a different body chemistry and have done more harm than good.
If your guinea pig is ill, please see a preferally piggy savvy vet!
If your guinea pig has stopped eating, you need to step in with syringe feeding and water asap in order to avoid the gut from slowing and then closing down and to buy your piggy the time necessary to see a decent vet and to allow any medication to kick in.
There is usually a pretty serious reason for loss of appetite that should be investigated by a vet familiar with guinea pigs, which the vet you have seen does sound like. Most often there is either a pain issue somewhere in the body (including bloat, gut stasis or some other source of pain in the body) or a mechanical obstruction (overgrown back teeth) in the digestive system.
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
Complete Syringe Feeding Guide
If you please added your country, state/province or UK county to your details we may be able to help you with finding a good vet. Click on your username on the top bar, go to account details and then to location; this makes it appear with every post you make and allows us to give you the appropriate information straight away. We are dealing with enquireis from all over the world.