Hi and welcome!
BIG HUGS
I am very sorry; an ovarian cyst operation is a major operation. It is always very upsetting when you lose a piggy in an operation. I know, I have lost several of mine, one of them under similar circumstances 7 years ago.

Please keep telling yourself that you have accepted the operation to give your girl a problem-free future. large ovarian cysts can create severe issues. My Iola has just come through a touch-and-go ovarian cyst spay as one of the cysts had got attached to the bowel. It could have easily gone the other way with any less experienced vet. Only two weeks later I unexpectedly lost Nesta who had had an abscessed incisor removed just after Easter.
You'll always feel a certain degree of guilt/failure when one of the pets in your care dies; it is normal for the onset of the grieving process. But this can be much stronger when it happens during or after an operation as you feel your responsibility stronger. It is not so much an expression that you have failed your girl, but how good and caring an owner you are.
You do not have to let your other girl see her friend if you have doubts that it could endanger her own recovery; she'll know anyway. There is no hard and fast rule. I do not always show the body depending on the circumstances.
You are welcome to post a tribute in our Rainbow Bridge if or whenever it feels right for you. We all grieve differently, so there is no pressure, but it is there for those who would like to make use of it at some point.
Once your other girl is recovered enough, you can consider to either bond her with another sow or a full boar - if you can get to one of our recommended good standard rescue, that would be great as you are allowing your girl to have a big say in who she wants to live with. Age is much less of concern that mutual liking and character compatibility.
You can find links to lists of good standard rescues via the bereavement guide.
Tips For Post-operative Care
Looking After A Bereaved Guinea Pig