How can I fill out an Incident Report if someone does not report the event until days, weeks, or even months after if happened?
Do the best that you can. Document what the potential claimant reported to you and note that you were not aware of the event until you received the recent report.
If the entity has records/documents/logs to verify or dispute the claim (i.e. we did not have equipment and crew at the location and time reported), please note and attach documentation.