A domestic worker would be expected to a clear contract with their employer. A copy of that contract would help.
You'd probably also want to help the domestic worker set up a bank account so that there is a clear paper trail.
Documents you must provide
When you apply you’ll need to provide:
a current passport or other valid travel identification
proof you can support yourself during your trip, for example bank statements or payslips for the last 6 months
a completed ‘Appendix domestic worker statement’ signed by both you and your employer
a letter from your employer confirming your job title, how long you’ve worked for them and that you’re a permanent employee
You’ll need to have a blank page in your passport on which to put the visa.
You must also provide 1 of the following documents covering the same period of employment:
pay slips or bank statements showing payment of salary
confirmation of tax paid
confirmation of health insurance paid
contract of employment
work visa, residence permit or equivalent passport endorsement for the country where you’re currently employed by your employer
visas or equivalent passport endorsement if you’ve travelled with your employer before
The "you" in the quote above applies to the overseas domestic worker, not the employer.
The
Leaflet for such applicants also specifies that a written contract, compliant with UK labour laws, needs to be signed by both the employer and employee and the employee is given one copy.
As somebody staying in the UK for fewer than six months at a time, they will not be able to use the NHS free of cost and you may want to look into private health insurance for them.
Also keep in mind that while in the UK, the overseas domestic worker would be entitled to the full benefits and protection of UK labour laws, including the UK national minimum wage, sick leave and holiday pay, etc.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.