Vets definitely are not created equal and it's worth seeking one that has experience with "small exotics." Early on in my piggie career I lost a pig who had a dental abscess. At the time I thought the care she received was the standard because I was an inexperienced owner and didn't know any better. Later on I realized that this vet had NO clue what he was doing. We may have lost her anyhow or she might have recovered with a more experienced vet- I'll never know, and all these years later I still have deep regret that I let her down because I didn't know better.
That said, misdiagnoses happen even with good vets... I had a pig in the same situation as yours, she was diagnosed with a malignancy when what she really had was a very large fluid-filled ovarian cyst. The vet was an exotic vet and she did do imaging (in our case it was an x-ray rather than an ultrasound, which might have been clearer as to what was actually inside the mass. In our area a lot of smaller vet offices don't have their own ultrasound machine, you have to go to a larger vet's office who may have less experience with guinea pigs, ironically.) Luckily we chose to take our girl home to live out her 'final days' with pain relief... she lived another 10 months before dying of unrelated issues at 6.5. The cyst basically drained out and she survived that, seemingly comfortably, because she was on tramadol the whole time! We still see that vet, although she's not my main vet at the practice. She did have the good grace to admit that she was wrong in her diagnosis when she saw Leela for followup imaging a few months later when the cyst was much reduced in size, so I like to think that Leela taught her something and some future piggie might get a clearer diagnosis because of her.
I would definitely let the vet's office know that they made a mistake in diagnosis, even if it doesn't go to the level of filing a formal complaint. If you know better, you do better. It might help a future piggie down the road and that's very worthwhile.
All the best for your piggie's operation and I hope she is soon back to her old self!