Unlike rabbits where spaying prevents very frequent ovarian cancer and is a real life saver, this is not the case with guinea pigs. Many older sows get ovarian cysts, but in the majority of cases they do not cause any problems and there are now much less risky non-invasive hormonal treatments available.
Spaying is a major, much riskier and expensive operation than boar neutering. We are now thankfully getting to a stage where the risk/benefit ratio in boar neutering is starting to swing to the positive side so we are feeling more confident in recommending it, provided a member has got access to a good vet. But I would certainly not recommend spaying yet without medical indication or unless you have access to a really good vet!
Whether your girl is spayed or not won't make any difference for your neutered boar. I have currently 4 neutered boars living with 2-6 sows each, but only two of my sows are spayed (adopted from a now closed rescue with a spaying policy; it was the only one in the whole of the UK).
Of all my elderly sows, only two ever needed a spay. One died during the op at my local vets four years ago. With the second I was lucky enough to have access to the same extremely practised vet that used to do the spaying for that rescue, so she came through it extremely well when her uterus went wrong; I doubt that she would have otherwise come through the op considering her low weight. It was well worth the travel.
PS: We recommend a full 6 weeks safety wait for boars after a neutering operation. I have had a surprise baby courtesy of a supposedly safe over 5 weeks post op boar (not one of mine), just to prove that particular point!
