Who are you to decide anything for anyone Jewish? Leaving one's faith or culture does make some parents go into mourning. I suppose you'd have a BBQ and kegger if one of your kids announced he or she was going Hare Krishna?
You are right; I would be grieving if my kid joined a cult, but there's a big difference between a cult and just joining another religion; adults do it all the time. My point is not to insult anyone Jewish. However, to equate the mourning for this with the mourning a parent would feel over a child who has died is really feeble.