Often times if a vocalist sounds pitchy it's smart to check their headphone mix. If the mix is loud it could cause them to lose pitch as they're singing beyond the note and overreaching.
Instead, advise them to turn it down and to focus on the music not their own voice. Tell them to sing within the music not at it. This challenge often helps singers relax more and sing on pitch as well as fit within the track better. It's a simple trick, but it works.