Hi and welcome
I am very sorry that you are in that position after getting the kind of diagnosis none of us wants to ever hear. If you are not happy, you are within your rights to seek a second opinion but please do so with an open mind and take on board any vet concerns.
Unfortunately, kidneys are still beyond successful operations at the time - we have seen the odd attempt by vets in younger guinea pigs have yet to hear of a successful operation. While I have had several successful emergency spays in 5 year olds and a much less invasive skin cyst removals in two 6 year olds, I would frankly consider twice a confirmed ovarian operation in a 6 year old who is right in the middle of the average life span.
I would personally be even more cautious about exploratory surgery in an older piggy and have always taken the advice of my vet in comparable situations. With internal operations where an internal 'mass' of some description was in play my own decision has always been a negative so far; especially when my experienced operating vet has not been confident about the outcome.
You have basically the choice between an expensive exploratory surgery that is more likely not to come off and may result in the decision not to let your girl wake up again if the findings are not good or letting your piggy live out their life for as long as they have quality of life (being able to exhibit normal behaviour) and the will to live for as long or short as that lasts. Neither is an easy choice but there is no right or wrong which way you decide as long as you are putting your piggy's welfare before your own desires and fears.
You may find a more in-depth discussion of this aspect (which also applies to concerns as to when to let a guinea pig suffering from a major health problem go) in this supportive and sensitive link here, which will hopefully help you to make your own choice with a clearer mind if not with a less aching heart.
Please also keep in mind that you are currently reeling from the bad news and that actually your own grieving process has kicked in with the loss of your girl suddenly becoming a stark reality instead of something that might happen some day in the future. The second worst time apart from the actual loss is the shock at the is the shock of it right at the start.
Here is the link, which will hopefully help you further in a constructive way to make as informed a decision as possible so you will have an easier ride later on, whichever way you go:
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
If you decide for an operation, here is practical information, which you may find helpful for the aftercare:
Tips For Post-operative Care
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Wishing you and your girl all the best.
HUGS