If you've decided to go, it's best not to drag it out. hand over to a carer so you know someone is cuddling her and say good bye and leave. Hanging around or coming back again can make it worse. It's ok to hang around together for a bit before leaving, I think, but once you're going, go (if that makes sense). Then call them a little while later and check on how she's going.

ETA - I would cut it short if she's hysterical and they can't calm her, yeah.