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Her friend is unfortunately talking about old domestic worker visa rules. These have changed since 6 April 2012 and it is no longer possible to extend or switch employer in that way.
6 months is the maximum permitted period for the visa holder to remain in the UK and she must leave the UK when the visa expires or when you leave the UK, whichever is sooner. If you remain in the UK longer than 6 months then she must still leave the UK before her visa expires.
This is confirmed on the following web page where conditions of the visa are outlined: