I am guessing she was meaning muriatic acid or Hydrochloric acid (HCl) this would sure clean the tiles, but so would other less hazardous products. Using HCl in a area like a bathroom sounds like trouble waiting to happen. I would not want to do this.http://jtbaker.com/msds/englishhtml/h3886.htm
The idea is to remove all traces of soap scum and mineral deposits. it the tiles are glossy they should best be sanded (try 3M wet or dry sanding sheets) use them wet and go with about a 100 grit perhaps. Don't use an electric sander when wet sanding.
There are primers designed to bond to glossy surfaces use one of these followed by a topcoat of a compatible oil or waterbase finish.
You might also look into 2 part epoxy paints. there are some used to refinish tubs that might work well. These are a bit more trouble to use, but they could well give the best results.
As with any painting job 3 words
Preperation, preperation, preperation.