Leave the tank lights off but leave a bit of light in the room, not enough to keep her up all night. Sick fish frequently pass if they are left in complete darkness, so just a bit of light. That is not to say that keeping the light on will result in her not dying, though.
I wouldn't risk the other fish, and there is nothing to say that transferring this fish now won't result in a lot more stress that she just can't stand.
Best thing to do is to plan on keeping an established QT for the future, to avoid this situation. It makes this hobby a lot easier on your nerves.
Here is the product specs for Stress Coat:
Quote:
Treats up to 946 U.S. gallons of fresh or salt water. Contains Aloe Vera which promotes healing and regeneration of damaged fish tissue and forms a synthetic slime coat to help prevent electrolyte loss. Stress Coat also removes chlorine, neutralizes chloramines and detoxifies heavy metals. The healing power of Aloe Vera: Replaces slime coat Reduces electrolyte loss Promotes tissue regeneration Removes chlorine and neutralizes chloramines When to use: When setting up an aquarium, adding or handling fish, changing water or when fish are damaged by injury or disease fish tank aquarium water care conditioner
Its not bad to use, but I'm not convinced that it will remove ammonia. I would suggest using the Maracyn Two for SW fish, but it doesn't sound like you have access to any.
Don't rush to do something or anything, as that could be worse. If this fish is too sick to make it, then there is nothing you can do right now to change that. Let the fish rest, or pass on, and you just take it easy for the rest of the night.