Black sink

Today, I was so upset.  You know why? When I arrived from school, the first thing I saw was our stainless sink which turned to black. My niece in her effort to clean our sink poured out Muriatic acid on it.  And you know how it looks now.  It's like a charcoal.  My heart sank when I saw it.  I pity our sink.  I don't want to even glance at it right now.  It really  disappointed me and ruined my day.  I wished she had asked or inquired before doing it.