Carly didn't completely leave when Papyrus began to speak with the Mother. She couldn't stand to look at Sans in this state of course, and the Mother's continued actions certainly made it worse, but she couldn't leave the area completely. Instead she quietly took to the air, keeping the skeletons in sight.

At one point, the water in the distance seems to be moving- and she almost wants to go in that direction, to see what's happening, how to help.

"Papyrus-!" She doesn't however, and her eyes move from the distraction of the store front fountain in time to see Sans' collapse.

"Sans-!" she adds a millisecond after calling the name of the taller one, floating down with an awkward flap to try and keep from flying too close or crashing. She may fail. But she's trying.

There is a pause...and she swallows, looking over the two.

"...She..." ... "...She's gone..?"