Ling is she in pain?
If not (and though guinea-pigs do hide things well, you'd know if an animal was in pain) just let her stay quietly in her house with her cagemate.
Guinea-pigs sort of 'switch-off' and go floppy, listless and starey (difficult to describe it but when my GP2 was ill, he just sat there. Not lying on curled up, just his feet neatly tucked in, eyes a bit dull, gently nodding, like he was preparing himself).They just close down.
I took our boy to the specialist guinea-pig vet but there was nothing that he could say "Ah yes, this/that is wrong" and I said I wanted him to die at home (I knew he was dying
which he did 7 hours later)
Do I regret taking him to the vet? No, of course not.
I do wish I didn't have to drive him 45 minutes on a boiling hot day.
It didn't alter his outcome.
I wish I'd just trusted my instincts and let him pass peacefully without the journey.
He wasn't in pain, just a bit annoyed at my attempts to feed him
. I only got one dose of antibiotics and probitics into him before he went.
6 is a good age for a guinea-pig.
Keep an eye on her, let her have peace and quiet, and anything she wants to eat.