Hello.
Chewing nails is a sign of anxiety and stress!
He can reduce his anxiety and stress levels by both - yoga & homoeopathy. Ask him to be regular with both.
Mental calmness is essential part for good focus, concentration and confidence!
Here are few yoga poses which helps in calming your mind and relieves stress, but first learn these under some qualified yoga-teacher as many poses are not to be done in certain diseases or medical conditions!
Balasan, Setu Bandha Sarvangasan, Uttanasan, Garudasan, Savasana, Triconasana, Viprita Karani, Marjaryasana, Dhyana, Bhujangasana, Anulom-Vilum, Pranayam, etc.
First, ask him to start with breathing exercises like Pranayam, Anulom-Vilum. And gradually switch to rest of the poses one by one.
Medication:
Take homoeopathic medication - Schwabe's Bacopa Monierri 1x/ thrice daily and Kali Phos. 6x/ once at night - take them for 4 weeks.
Revert back to me after a month.