Standard Visitor visa - 6 month calculations

[ninok]

Hi All,

Hope you will be able to help me. I live in UK under Tier 2 for 5 years, my mother has Standard Visitor Visa for 2 years and visited UK in November 2016 for 10 days (hew only visit in UK).
She plans to visit me again in April for another week.

Every time we enter UK, boarder officer add a stamp in the passport with the date of entry, however when we travel from UK, there is no stamp, i.e. passport does not have any information when my mother left the country.

How boarder officer will know that she was in the country only for 1 week, did not breach 6 month rule during the first visit and hence can enter UK second time?

Thanks
Nina