Be Thou My Vision is an Irish hymn, but it doesn't have the same tune as Come Thou Font.
Be Thou is the Slane hymn tune and Come Thou Font is the Nettleton hymn tune.
If your friend has a hymnal of most any denomination at home, and it turns out the song she was thinking of isn't Be Thou My Vision, have her look up Nettleton in the hymn tune index in the back of a hymnal.
Hymns aren't very original when it comes to tunes. Many of the hymn writers would write new words to familiar tunes already written. This was done so much that just about every hymn in a hymnal will tell you what "tune" it is. (melody) What funny is Martin Luther and some other prominent hymn writers of old, would use drinking songs sung in pubs to write their hymns. This was to try and attract the attention of the ordinary citizen.
Sorry if this is too much info. The church music director side of me took over!