Thought I would give a little update on the outcome of the issue.
I put a fatigue policy together to be reviewed and approved by management and in the interim the other company that was employing them let them go due to H&S risk.
Still waiting on the policy to be approved and implemented so we do not end up in this situation again, which with the cost of living feels a bit inevitable.
To those that suggested a pay rise might help, you are right it really would, but not within my control.
Thanks for all the advice and suggestions.

Sorry, new to the site and pressed share by accident, thanks for the article really helpful

The trouble is there is no clause in contracts about second jobs, and it seems everyone has a 'side hustle' these days it has made we aware that we don't have any protocols around this.
Working two full time jobs is unsustainable, so not sure how to manage it if the employee does not see it as an issue.
Waiting for signs of impairment/fatigue does not seem responsive enough, can it become an immediate HR issue without signs of impairment / fatigue?