I'm usually someone who agrees with Lord Sugar's decision, basically because it's his show so he can do what he likes.
However, knowing that the only reason Selina got her sale was because of Scott, it kind of put Ruth and Selina on the same level in terms of performance in the task. But then if you take into account the general attitude, Ruth seems to be pleasant company while Selina is moody and easily gives up.
Therefore I would have fired Selina, or at least not sent Ruth away on her own.
Also knowing about this weird drama between Selina and Charleine, if that physical encounter has happened and Selina got a warning, then she really should have gone.
I presume she's being kept around for drama.