Can UK employer ask non-UK candidate about their marital status?

Question

The company in question employs some people outside UK, I overheard a conversation with a manager and external staff where manager asked, maybe jokingly, to investigate potential staff's marital status. I know it's illegal to do so in UK, but considering the employer is UK based and the candidate is not, do UK hiring laws apply or would the native's country's hiring laws apply?

Edit: To clarify since there's some confusion:

The company and manager are UK based, the law for hiring UK residents states that is illegal and it is generally unethical, we're on the same page. However, considering that the candidate is not located in UK, and that they would work in a non-UK office, is it technically illegal?

Answer 1

No, they are not allowed to by law as stated in the official gov-docs since they are an UK-based company and the recruitment is done from there (as you stated in your question), so local laws do apply, regardless of the nationality of the applicant/candidate:

Discrimination during recruitment/Discrimination in job adverts

You must not state or imply in a job advert that you’ll discriminate against anyone. This includes saying that you are not able to cater for workers with a disability.

Only use phrases like ‘recent graduate’ or ‘highly experienced’ when these are actual requirements of the job. Otherwise you could discriminate against younger or older people who might not have had the opportunity to get qualifications.

Where you advertise might cause indirect discrimination - for example, advertising only in men’s magazines.

Questions the employer cannot ask when recruiting:

You must not ask candidates about ‘protected characteristics’ or whether they:

• are married, single or in a civil partnership
• have children or plan to have children
• Asking about health or disability

The employer can only ask about health or disability if:

• there are necessary requirements of the job that cannot be met with reasonable adjustments
• you’re finding out if someone needs help to take part in a selection test or interview
• you’re using ‘positive action’ to recruit a disabled person

Link

From comments:

"That confirms the law in UK, but I don't see how it implies that it applies to non-UK applicants and adverts?"

Answer in comments:

"It does because the company operates in the UK and therefore need to obey to UK laws. Lets imagine following scenario: In the UK it's forbidden to steal candy from your local grocery - does that law only applies to UK-residence? No - it applies for everyone stealing candy in the UK.."

In case companies are breaching employment rights in the UK, substantial penalties could be fined by the government (see reference). If that's really the case the company should replace that manager asap as stated in @Gnasher's answer before such fines could be imposed onto them.

Answer 2

It is illegal in the U.K. The reason why it is illegal in the U.K. is because it is unethical and undermining the values of the British society.

If your manager in the U.K. trumpets out that he wants to do things outside the U.K. that are illegal in the U.K., unethical, and against British core values, even jokingly, then the company should consider replacing that manager.

Whether it is legal elsewhere doesn’t matter.