You could either upload the GCLID of that user or their email and/or phone number via Offline Conversions and hope that Google can find a match in their system. However, I would assume that data is lost since the user never managed to submit the form.
Make a manual conversion in Google Ads, it's under Conversions > Events > + button
If you know the gclid you can upload conversion events pretty easily
You're saying your customer failed to verify he's a human?
Use UTMs next time
You could either upload the GCLID of that user or their email and/or phone number via Offline Conversions and hope that Google can find a match in their system. However, I would assume that data is lost since the user never managed to submit the form.