If you did the Covid extension on top of a tourist visa they may question you at immigration control as they will think you are not a real tourist and have a job here
All her bank statements showing wages going in, letter from employer, dates arriving in UK and leaving, all her family details (mother, father, kids ), if she owns her house then chuck in the deeds. Amount she will spend when in UK, if supporting her ALL the transfer slips from western union etc, chuck in your own bank statements showing you can support, copy of your passport, anything else you can think of.