HUGS
That's a really difficult one to call, and - unfortunately for you, we cannot make that decision for you; only you can do that. However, before you make any decisions you need to clearly know for sure which piggy the blood is from. If needed, isolate them for a day, so you hopefully get that sorted as your first priority. Your piggies' bond will survive that.
However, recurring bleeding from the same guinea pig is worrying. If it was from the urinary tract, it would be more likely mixed with urine. Because a larger, rough stone is very painful when it scrapes its way down the urethra, removing the insulating glucosamine layer that prevents stinging pee from coming into direct contact with raw tissue, it is usually very painful and your piggy would clearly look off - at least in my very own experience. The pain in the bladder or urethra is unmistakeable.
The bloody area is clearly not from your classic urine infection (bacterial or sterile), as that would be porphyrin (dark red) coloured pee drops and puddles.
Your vet should have been able to feel a stone if they palpated (felt for) the bladder. If the bladder is soft and large, then it is healthy. It sounds rather like your vet has been able to exclude the urinary tract as the source of the blood.
Bleeding from from the reproductive tract of a boar is very rare - it means an injury to the penis or something sharp stuck in the penis pouch, and that is rather easy to check and feel.
As to the operation dilemma: any diagnostics are going to add a few hundred of dollars more to the bill but you
are entitled to ask for further examinations.
A spay is a major operation that may or may not come off; it depends on the operating vet, the post-op recovery team and the reaction to GA.
However, you have to weigh that up against the possibility of the bleeding coming from a cancerous ovarian cyst or the womb going wrong - both with usually fatal consequences if it was that. Pyometra (infection of womb lining is fatal in the advanced stages but it could also be cancer in the womb.
We have seen several cases of cancerous cysts in sows of 2 years onwards. They have ended with a death not long after.
It is one of these 'between a rock and a hard stone' decisions that we all dread because there is no peep into the future for the outcome, so it is very much a leap of faith in the dark whether you want to risk a make-or-break operation or prefer to just let her go on and then have your piggy euthanised when they start going downhill and are clearly in pain.
This guide here can help you work out where you stand on important aspects that are playing into your decision for yourself:
Operation, Terminal Care or Euthanasia? - Helpful Questions to Ask Ahead or in Hindsight
Tips For Post-operative Care
I wish I could just snap my finger and pull out a safe treatment option but sadly I cannot. It is even more difficult when you feel that you have not enough clear information to base your decision on.
My thoughts are with you.
