Musings on Sharing

How do you tell a relative, one of the major family genealogists, that you just found a batch of new cousins for him? A relative connected to the last sibling I lack in a group of four, somehow left off our family tree until now, contacted me, and was able to confirm the relationship, making that grouping complete. How do I tell my poor cousin about this, though? Although I've tried to let him know some of the facts, I have a bad feeling that he still thinks that the relative this group of four children belongs to died in childhood. Do you suppose I could be blamed for giving a relative a heart attack when I present him with his 40+ new relatives?