Basically, your water isn't going to get unconditioned. It won't hurt to leave the water out for however long, though I would suggest keeping it somewhere where dust etc can't get into it. The only disadvantage to it, of course, is that the water will be room temperature when she gets to use it, instead of the nice toasty tank temperature that you can get from the tap.
Bettafix, Maroxy and salts are not water conditioners but treatments for sickness and should only be added when the fish has the specific disease that they are supposed to treat. Remember not to treat without knowing what the disease is - many illnesses can be cured with just clean, warm water. :)