Hi and welcome
You have not killed a guinea pig that was already in sudden acute heart failure. This is a genetic time bomb that can go off at any time in guinea pigs who have inherited bad genes. I have lost two young, seemingly healthy piggies this way myself. It is a very frightening experience. The good news is that is that you are not dealing with a contagious problem.
It is very normal to have strong feelings of guilt after an event like that. We are wired to seek the fault with us. Any loving pet owner experiences feelings of guilt or failure at the onset of the grieving process, but the circumstances of a death can intensify this. Please don't feel guilty!
Please bring her companion indoors if you have outdoors piggies (it is the season for it anyway to bring outdoors piggies under cover for the winter). See how she copes. If she stops eating and drinking, then you need to act fast (acute pining or illness); if she is hanging in there, then you have ideally about 1-4 weeks to organise new companionship. Your girl is likely very withdrawn for a few days as she is grieving. As long as she is still eating, she will come out of it again while you are doing your research.
The safest place for fiding a new friend of your surviving girl's liking would be by dating at a good standard rescue under expert supervision. With sows you can look at both sows or neutered boars of any age. Mutual liking comes long before either aspect!
By rescue dating at a rescue with mandatory quarantine/vet care, pregnancy watch and neutering post-op wait (ideally 6 weeks) you can also be sure that you can neatly avoid all the usual pitfalls and bring home a healthy piggy of your bereaved girls' choice.
All the best with finding a new friend! It usually pays to contact all rescues you can get to and then start with the one that has the largest number of potential candidates for dating.
The RSPCA Middlesex Northwest branch also have guinea pigs from time to time but they are more limited in what they can do (only a few places will offer dating) and their piggy savviness varies quite a bit from branch to branch; but the RSPCA have a mandatory quarantine, pregnancy watch and a mandatory neutering policy with 6 weeks post-op wait. It may however be another place for you to contact if the two closest rescues haven't got any suitable piggies available at the moment.
Middlesex North West & South Hertfordshire Branch - Home - rspca.org.uk - RSPCA
You may also find this link here helpful for yourself and your family:
Human Bereavement: Grieving, Coping and Support Links for Guinea Pig Owners and Their Children