The employee is in trouble, in big trouble. Having a blank form with the signature of a boss is obviously a capital case.
But you are also in trouble, because it couldn't have happened without your cooperation. If you are allowed to make financial decisions, or at least influence them, then doing such a thing is also a capital case in your situation.
Even if you would be a simple employee whose only financial decision is to say his (her) bank account number to the HR, it would be dangerous.
At the moment as you signed that paper, you committed some hard thing and now all you can do is to minimize the possible casualties.
Of course, if the signature on the paper wasn't made by your hand, then you are clear and the employee is in really big trouble, but for your question, it doesn't seem that it would be the case.
What you can do: do it in a setting as if a trivial administrative mistake had been happened, for example you had printed not a correct form but another. Ask for the paper from the employee and handle it as a simple bad printing (-> destroy in the office document shredder).
Because he is also in trouble, he has every reason to be 100% cooperative.
The best result: nobody will remember it after some weeks.
The worst case, if the problem escalates to your bosses, a direct action of your bosses is likely, which can be followed by silent firings. If no actual financial loss (= forged purchase) happened, a further escalation into the criminal law is very unlikely (nobody wants it).
The goal is the best result. Try to do everything in the "business as usual" frame.
If you have similar tasks with the same employee (for example, other purchases requiring consultation with you), then do some other, similar tasks concurrently with him, and also solve this minor "bad printing" between them.
If not you and the employee are the only ones in the company who know of this problem, it is much more likely that your bosses will also know it, even if they won't say it to you.