I am not entirely sure where this idea that any exposure to air will kill an otherwise healthy seastar. In and of itself, it will not kill them. I've held them out of the water to show students and to examine them, however, I didn't do this for too long. Sometimes seastars can be exposed by low tides for a bit too, so I wouldn't write him off yet. I mean, you weren't holding it out of the water, so it could have moved itself too.
HOWEVER, if there was a significant difference in the water you then added, in terms of salinity, temp, pH, etc...it is possible this really stressed it out. But I am not certain. Only time will tell.
If he starts to "melt" then this is a bad sign. But we will cross that bridge when we come to it, it is not guaranteed.
How long have you had the star, and what are your tank specs (eg size, LR and specific gravity?).
FYI, there are a few practical ways to 'humanely' destroy echinoderms. One is a reverse acclimation, where fresh water is added to a container with the seastar and a bit of tank water. This is literally a reverse acclimation and done slowly. Eventually the tube feet will no longer respond, and this is a sign the animal is dead. You can put it in a bag with some water and freeze it as well. A final way is to put about 2 tablespoons of Epsom salt in a corner of the container with some tank water. Every 5 min or so, you can tilt the container to slowly mix in some salt. Eventually, the animal will no longer respond.
