If you choose to tell more, keep it to a minimum. Stick to the facts. Now is not the time to speculate about your former employee's bad qualities, or to opine on the reasons for his failure to perform. Limit your comments to accurate, easily documented information. Don't be spiteful.
Many states offer some protection for former employers called upon to provide a reference.
These laws generally provide that you will be shielded from defamation lawsuits as long as you provide information in good faith. This is a fairly nebulous legal standard, but it surely does not cover nasty or mean-spirited gripes. Don't give false flattery. You may choose to give only name, rank and serial number, but if you give a more expansive reference, don't hide the bad news. You may find yourself in a lawsuit for failing to warn the new employer about these serious problems.