From the Super Mario Wiki, the Mario encyclopedia
Mario must head to the west side of Rogueport to find Frankie next to a fountain. When talked to, Frankie tells Mario that he lost his Wedding Ring in Rogueport. To recover the Ring, Mario must head to the east side of the town and cross the stream near Garf's house with Mini-Yoshi. He must then return the Ring to Frankie, who rewards him with a Gold Card, and the trouble is solved.