This ballad is usually called "Lady Margaret"; it's in Child's ballad collection as #74. There are two versions in the Digital Tradition database: LADY MARGARET 2 and
LADY MARGARET AND KING WILLIAM

Margaret and William (or Willie) are found in a number of different gloomy ballads; as lovers, they never seem to wind up happily ever after. In "Margaret and Sweet William", it's William who dies and comes back to Margaret as a ghost; in "The Douglas Tragedy", they run away together, but her father and brothers hunt them down and kill him; she dies of grief, etc...

I don't know Walter Forbes, but if you look up versions of Child #74 in different folk song books, you might find the particular set of verses that he sang. Ballads changed and evolved as they went from place to place, so there's bound to be many different versions of the same song. Happy hunting!