The wart thing is most likely just that... a wart... or a cyst. A dr. will sort that out for her.
As far as the yellow... I would be very surprised if it was the bed. Most likely just the plate.
Many nail enamels can stain the plate and make it turn yellow. Using a basecoat is best to prevent this.
Also, formaldehyde strengtheners can also cause the yellowing.
Hope this helps