Cheating Situation-Helpp!!?

The names in this story are changed for protection of all parties involved. Cindy, Paul, and Mary went on vacation. Mary and Paul have been dating for many years. At some point in their relationship Mary cheated on Paul, and felt no guilt for the situation. On their vacation Paul made ual advances on Cindy when Mary wasn’t there. The guilt from what happened is killing Cindy and Mary has no clue as to what happened. Should Cindy tell Mary what happened? Answer and Explanation Please-Thank you