Can they tell when they're off, or do they think they sound fine? And do they have rehearsals, or do they just wing it?
If they know that they're struggling, maybe you can give them some guidance. The harmony parts on songs usually follow the chord progressions that you are playing. If the soprano voice is singing the melody, they'll be hitting a note that's probably in your chord. The alto voice would take the next note down in the chord, and the tenor voice would take the next note down from there.
If they can't tell that they're off and you're not the person in authority, that is a harder situation to deal with. It would depend on the spirit and character of the Minister of Music and how open they are to receiving help.