Thank you. The prayer is to be sent in a card, although someone else told me that sympathy cards are unheard of in India. The son and his wife are my friends, he was born here so is essentially Western in his ways.
"I am so sorry for your loss" should be fine I should think. No need for any special references to Krishna or anything else. White is the colour of mourning though black is fine so a white card or card with that colour combination should be ok.