To be fair, a lot of contracts have a clause to confirm that you don't have any unspent convictions. Granted, you can lie and indeed it probably will never be discovered but that's fraud.
> The Supplier warrants:
> that the Consultant has no criminal convictions which would reasonably affect the Company's and/or the Client's decision to allow the Consultant access to the Location(s), the Client's Systems or to provide the Services;
Depends on locale. In the UK (IANAL) it wouldn't be _fraud_, but would be grounds to terminating the contract. But not criminal.
Employer could potentially sue for damages, but there must be some damages for that.
After Brexit he'd need a work permit for that, and I assume convictions are a factor with those.
And that may not be fraud of you have a lawyer analyse it.