The great thing about oil and water is that they dont mix. So all the oil that might have been on the starfish or the water that got in there is going to be on the surface. Which can easily be pulled out by putting a powertowel on top of the water.
Furthermore, many of us have an oily shean on top of the water. It simply means you need more flow on the surface. So at worse that oily sheen is blocking gas exchange, which is a serious problem but not a toxic issue.
If there are some sort of toxins in the water doing water changes is going to dilute it out pretty quickly. It may not remove it overnight, but knowing Karen, im sure she has done regular water changes and anything is now at manageable level.
Also, if starfish are so much more sensitive to toxins and such, and you have one living happily in that tank - why cant a fish live in there?