My Husband Has A Square Shaped Rash On His Side

My husband has a square shaped rash on his side over his ribcage. He went to his doctor and was given a cream to put on it about a week ago. He completely forgot about it and never used the cream. Today the red rash now looks darker. Kind of grey. Also, it's peeling and he says it feels like there's fiberglass or splinters in the area. When I found out he had forgotten about the cream I had him take a shower and I put some on him. He says it still feels very painful. Is this something he needs to go back and get checked or should he just continue to use the prescribed cream?