I believe he wants to find the third of the melody note he has sung so that he can do Beatlesque harmonies BUT also wants to hear the notes so he can sing them.
Mario's suggestion is the quickest work around as a) Melodyne will achieve a decent result of it's own accord and b) the results could be used as the template to sing along to as desired BBBUUTTT if the fellow can't work out thirds for himself them using the harmonizer and singing along to that, as mentioned, is the next best thing.
Then again he could work out a counter melody type harmony based on the chord tones but I doubt that's an option.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe