Find out how many surgeries the vet has done. Part of outcome depends on the skill of the vet, post operative care, general current health of the guinea pig.
That there is a urine infection implies the possibility of a uti or stones. I take it these were ruled out?
A uterus infection (pyometra) is indeed serious and a spay is generally done.
How healthy is she right now? Have you been weighing her daily to ensure she is eating? What signs were you seeing that you saw a vet?
The vet did an ultrasound and needed to consult with an exotic vet to take some opinions on how would the vet proceed. But according to him, the ultrasound showed both an infection in the uterus and a urine infection. And the blood that was coming out of her was a lot, so I think its a very serious infection. The vet gave her an injection and she stopped crying, and ate a little bit of lettuce and cucumber. Nothing else, this morning the food, hay and water was untouched..but I gave her some lettuce which she ate. Today I need to go to the vet again for another injection and he will tell me how to proceed with the operation.
The problem is she is a small guinea pig and I am scared that she will not handle the operation and die afterwards. I don't want to let her undergo the stress of the operation and having her die a few days later. I wish to know what is the probability that the operation will be a successful one.
Also, you must hand feed right now.
I would be concerned if there is excessive blood loss. You would likely see weakness if there was.