sometimes the best approach is to simply be honest and state your opinion that the vocals just aren't cutting it
the studio can be a very humbling experience place as the recording media whether it be tape of digital has no preference, sympathy or compassion- if there's pitch problems, it is what it is
Pitch correction .... sometimes I think this was an evil invention, allowing folks to 'slide by' the honesty of the music- a little pitch correction is usually not too noticeable, but it sounds like you're already accepting the fact that it's gonna take some work- sometimes the old say "you can't polish a turd" has more relevance and irony than you'd like ... but....
I'd suggest that your friend go home and practice and do it looking into a mirror- not because he/she needs to see themselves but it keeps their chin up which helps to keep the diaphragm from being compressed, so breathing is much better. Breath is the most common culprit
or re-work the song, changing to a key they can sing
If they think they sound great, bounce the song and let them deal with well meaning friends and relatives- who know's could be the next Yoko Ono?