The issue here is not about your right to say "No," it's about how you're going to say "No" and who is going to be present when you explain why. (Accepting her as an authorized signer, not an owner, means simply that you think one form of poison tastes better than the other.)
While it could be the favor of a lifetime if you were to let him know that his intended is a deadbeat, it doesn't really fit in your job responsibilities. Hopefully, you will run her through a third party service at the time of her application, they will tell you what you already know, and you can hand her the appropriate notice. If not, you're going to have to do it the hard way based on information from your records and make certain she's the only one who hears what you're going to say.
_________________________
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.