Whose song was it? The mariachi band in Plaza Mijares

Three musicians were warming up in Plaza Mijares on a Thursday evening when a man on my bench began to cry quietly during the second song. His wife used to sing it, he told the bench. Nobody said anything, including me. Someone passed him a napkin. When they finished he stood up, thanked the trio in a low voice, put a folded note in the guitar case, and left. I think about that song every time I cross the square. I do not know the title. I have not asked, in case it is his.