rubbing salt in (someone's) wound

Translations
сыпать соль на (чью-то) рану
Telling her about his new girlfriend is just rubbing salt in the wound.
Рассказать ей о его новой девушке - это просто сыпать соль на рану.
Definitions
To make someone feel even worse about a painful or difficult situation by reminding them of it or making it worse, similar to making an injury more painful by putting salt in it.