What is the best way to teach a friend to sing? (Improving hearing ability)

One method could be a simplification of one I use to write large vocal parts - as even after all these years & can't just write them all down, or even sing them all one after another straight out of my head.*

First, pick a simple song, a 3- or 4-chord wonder to start with. Maybe a country-style song - something with a strong melody but nothing too 'clever' in the way of melodic structure.

Play simple chords left hand & the melody right hand - which will give the melody something to sit on, rather than being free-floating with no reference.

Emphasise which note of the chord references which melody note, then point out any that aren't part of a simple triad - pick tunes where these are more passing notes, rather than something that will to a beginner appear to just sit entirely in the cracks. Stay away from complex or chromatic structures initially. This should start to train your friend to be able to get the first note right just from the chord - a necessary skill, ongoing.

That should give them sufficient grounding eventually that you can progress to something more complex.

*I extend this method when writing so that I'm playing the existing parts, leaving me to sing the new one until I'm happy with it. I then swap which to sing & which to play until I've got them all, before I even record the first part.