I think it hinges on whether your daughter wants to do the ex-hubbie-still-a-dubbie a favor or not. The letter they are asking for is likely for use to document the end of one marriage so he can have another.
If dubbie-ex-hubbie was a jerk, then go ahead and ignore the request for a letter. But if you want to cause him some trouble, just reply that you would like to live up to your responsibility and take him back. Then the elders wouldn't allow the ex's new marriage.
If he was a decent guy, you could send a letter so he can get on with his own, even if is is misdirected, life.