Been given a bubble eye from a friend which is going to be the basis of the stock of my sisters coldwater setup. However, her tank is around 50L which im aware is pushing it a bit for space. But i'm sure with good filtration and regular water changes this will be fine.

But my question is, will the bubble eye be the only fish she could keep in the tank, or are there other companions she can add in a tank of this size?

Bubble eye goldfish are the smallest growing variety of goldfish and tend to max out at 5inches long, a 50litre tank is fine for bubble eye goldfish (it depends on the measurements of the tank though, a long tank is better than a tall one), if the filtration is good (goldfish tanks need to have strong filtration since goldfish create a lot of waste) then getting one more bubble eye goldfish would be good depending on how long and wide the tank is .