Was the abscess left open? A lot of the time you will need to continue squeezing out any pus and irrigating/cleaning the wound. They can take a long time to heal from the inside out. Stitching an abscess closed is not the best way to allow it to heal (unless it was removed completely. But if it was opened and irrigated, it's best to leave it open.) Guinea pigs have very thick pus and abscesses generally don't clear with antibiotics alone.
I've been through a couple of abscesses... basically on a daily basis I would get cotton pads and something to irrigate with (saline or antiseptic rinse from the vet generally.) I would use a syringe to irrigate inside the wound, and the use gentle but firm pressure to squeeze out any additional pus. Over time, this allows the abscess to from the inside out- but it can take quite a bit of time, depending on the location and depth of the abscess.
What antibiotic was Peppa given? My understanding is that Zithromax is the gold standard of clearing abscesses, but it's not commonly prescribed in some places (I'm in Canada and cannot find vets that give Zithromax, including exotics vets.) Baytril is commonly prescribed by vets who don't know what else to give, but there are other safe antibiotics that have better track records with abscesses (my vet gives chloramphenicol for abscesses, for example.) As for long term antibiotics being bad for guinea pigs, personally I would dispute that. One of my pigs is on lifelong suppressive antibiotic therapy due to bladder issues. She has been on a low-dose since about the age of two and is now turning five next week. She was also on chloramphenicol as well as Baytril for months following her abscess surgery and did very well with it.
Does your vet have a lot of experience with guinea pigs? The antibiotic comment and the fact that the abscess wound sounds like it was stitched closed makes me wonder how often the vet has dealt with this kind of problem. Depending on where you are, it can be hard finding vets who have experience with guinea pigs, but if you are able to find one, it's well worth your time and will increase the odds of your guinea pig making a good recovery.
Hope this helps a bit!