All she does now is sit in her hides and mope and she is so young that it'd probably be cruel to keep her by herself.Personally, I would not let a guinea pig live that long alone. Guinea pigs are highly social animals. While they can do fine alone, they are far happier with a companion. Often when guinea pigs don't do well together, specially for females, they don't have enough room or proper hiding places. There are so many people who get guinea pigs for their kids and end up rehoming them this time of year. It would be easy to get another.
If the guinea pig was old and may not live much longer, I wouldn't consider another guinea pig unless you were planning on continually having guinea pigs.
I had one guinea pig who was unable to live with the others for about a year. She became suddenly a terror. She then became sick and was starting to make a recovery. She wasn't as strong so we put her back with the others. She got along great and was the happiest she had been in a long while. So even though they can live alone, they will not be as happy.