I read that they need a grieving period but when I spoke to a breeder she said that IHO leaving them alone can also be a bad idea.
Our Guinea pigs were two girls and they went everywhere together. The dominant one has died and now that she's alone, the remaining GP just doesn't seem to know what to do with herself. She is eating fine, however. Dd put her toy GP in her cage and that seems to have soothed her somewhat.
If we introduce another one would sooner be better than later?