Burd Helen, heir of Broughty Walls, is being visited by her beloved when she is abducted by armed Highlanders. Her kidnappers try to console her, but she refuses comfort. At her first chance, she swims to escape, while one who pursues her drowns
It appears to me that Child has misinterpreted this ballad (which seems to be missing a key stanza or two). He believes that, when Helen is kidnapped, her love Glenhazlen follows, and it is he who drowns when she flees across the river.
There is, however, no evidence in the text that her lover followed; all we know is that, after she threw herself into the water, "It was sae deep he [who "he" is is unidentified] couldna wide, Boats werna to be found, But he leapt in after himsell, And sunk down like a stone." Also, Helen rejoices at her freedom following her escape. So it sounds to me as if one of the Highlanders (presumably the prospective husband) is the one who drowned, not Helen's lover. - RBW