Hi
I am ever so sorry for your loss.
Here is our comprehensive neutering information, which you will find helpful. A lot depends on the experience of your vet with small animal ops and the post-op nursing team:
Neutered / De-sexed Boars And Neutering Operations: Myths, Facts and Post-op Care
The alternative option (and one you can implement straight away) is to put your piggies in adjoining cages or a divided larger cage so they can interact with and stimulate each other. You will be facing a 6 weeks post-op recovery wait anyway as he operation removes the testicles but not the tube that carry the semen to the penis.
Looking After a Bereaved Guinea Pig
Single Guinea Pigs - Challenges and Responsibilities (contains chapters on bereaved piggies and your various companionship options)
You can never predict how a bonding goes. In the end, it always comes down to a personality match and whether the two piggies involved want to be together or not at the bottom line. There is no measure that can change their outlook, how the two personalities vibe and predict the dynamics between the unique personalities involved.
Here is some more helpful information about mixed pairs and bonding dynamics:
A Closer Look At Pairs (Boars - Sows - Mixed)
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Please take your time to think things through. With a neighbour through the bars, your bereaved girl is not likely to pine away. She may (or may not, as piggies can react very differently to a death) be withdrawn for some days as she is grieving for her mate. Please give her that time; as long as she is eating and drinking, she will be OK. Guinea pigs don't grieve any less deeply than we humans, only that the urge for survival is allowing them less time to do so.
And finally, here is our practical and sensitive guide for grieving humans; especially when you have never been confronted with a death and a loss before or have children:
Human Bereavement: Grieving, Coping and Support Links for Guinea Pig Owners and Their Children