I wouldn't be considering putting her to sleep until all options have been explored! Do you have a more experienced cavy savvy vet local to you? Try looking on the recommended vet locator on the top left menu bar, there maybe a vet that's more suitable than your current one!
As I said above in my previous post, see if you can get the antibiotic 'Zithromax' prescribed it worked wonders for my old girl Grace
You may need to sign a disclaimer as I think it's licenced for humans but not guineas, but many members have had success with treating abscesses with it.
Its good that your Guinea has the dog Metacam as that's the stronger variety compared to the cat one, are you splitting the dose into 2 even time slots? Eg 8am & 8pm? This is usually more effective than the whole dose once a day, as their metabolism is so fast it wears off before the 24hrs are up.
Keep going with the bathing/flushing as any infection left inside will be trapped inside if you stop too soon, this will cause another abscess
Keep up the good work, these abscesses are difficult to get rid of, but it can be done - it's easier if the teeth aren't the cause of it too
My Grace had hers marsupialised(stitched open), I was flushing it with baytril, giving her Zithromax orally & it was about 6wks from start to finish, but as soon as her hair grew back you would never have known it was there
Make sure she keeps eating to keep her teeth & digestive system in good working order.
Definitely get a second opinion from another vet, I personally wouldn't have any faith/trust in the first one. Xx
Healing vibes being sent your way for a full & speedy recovery xx