Visitor Visa Extension -Accepted Reasons

Dear Forum,

I have a friend and her young son arrived in the country on a 6 month family visitor visa.

What are acceptable reasons that the UKBA will accept for the application to be successful for an extension on her visa. She is worried that she will lose her money if the application is declined so wants to know in advance.

Does anyone have any experience or been through the process?

Many thanks in advance.

As far as I know its under exceptional cases like family member illness etc. It is not a very suggestive course in my opinion unless dire emergency. What's your circumstance?