Since the norm is infrequent contact and I haven't seen my friend for years, I'll send a Christmas card with a note mentioning my concern. It would be difficult since this is the first Christmas without the surviving parent.
Hi: I think I would send a Christmas card with a note saying how sorry I was to hear of her/his mother's passing and how it might be difficult for the family this holiday season. Let them know your thoughts and prayers are with them during this time.