HUGS
Please try to take comfort in that her journey has gone fairly quickly (by the sound of it was one of the easier ones as these things go) and that she could make the journey from her familiar surroundings. I am always relieved when one of mine doesn't have to be rushed to the vets for an emergency euthanasia because the end is not coming as kindly and quickly as I'd like.
Be kind with yourself and give yourself time to digest it all as you haven't had time to brace for it all. What has happened is no fault of yours; it is just the much faster metabolism that guinea pigs turning against them at the sharp end. Once one major organ or the immune system gives way, the others will follow much more quickly than in larger beings. There is nothing you can do once that process happens. It is the quicker the frailer a piggy is.
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children
Give the companion a few days to do their own mourning as long as they keep eating and drinking; they will be aware that their friend has been very ill. You can leave the dead piggy in the cage for about 30-60 minutes unless the companion takes their leave sooner; piggies that have already said goodbye won't necessarily feel the need to do so again.
It will be hard to watch for you seeing a piggy withdrawn but they do not feel any less deeply than you and they deserve to have their own feelings respected. You will know when they pick up life again; some piggies will just plod on as if they were not affected by the loss.
What you do afterwards for the longer term you will have to figure out in the coming weeks (about 1-4 weeks is ideal but if it takes longer to organise a special companionship then it is worth waiting); whether you want to start with a pair of young piggies either as live-in companions or - if your remaining sow prefers to have her own territory as older sows not rarely do - as neighbours across the bars for the still needed stimulation and interaction. Alternatively, another bereaved old piggy as a live-in companion or neighbour may be a solution for you ending your guinea pig journey. If they are not living together, the gender is irrelevant as long as they cannot get together.
Here are lots more very practical tips and information:
Looking After a Bereaved Guinea Pig
Single Guinea Pigs - Challenges and Responsibilities
Here is some more helpful information for looking after older piggies:
Caring for Older Piggies and Facing the End - A practical and supportive information collection
For new piggies:
Rescues (Adoption and Dating), Shops, Breeders or Online? - What to consider when getting guinea pigs
I hope that the information links with the much more detailed in-depth information and practical how-to advice will help you with the practical challenges and considerations you may encounter over the coming days and weeks. Take the time to wait with any decisions and research until your head is clearer and make sure that you also look after yourself and get some much needed rest and sleep before anything else!
