Treat the worker fairly and justly. Put him at ease.
Be aware that one human error easily leads to another unintended human error. Relieve the worker of his current tasks if possible.
Make a mental note of the situation or take notes. Note the time. Take an objective look and record for yourself WHAT happened and HOW. Talk to your colleagues about the safety event. What was involved? Avoid any signs of early judgment on good and bad actions. Try to describe as accurately as possible the circumstances of the event: people, systems, procedures, environment. Note any production pressure that could be present.
Treat the worker fairly and justly. Put him at ease. He did not start out to deliberately make errors. Remember the organization selected, hired and trained him so it has an automatic accountability in the event.
The organisation wants to learn WHY it happened so they can prevent it from happening again; or prevent your colleague falling into the same systemic trap. No blaming and shaming should be involved.