if you knew she wouldn't be leaving straight away, you could have asked the embassy for the visa to start on a later date, but you can also try and ask the immigration officer when she lands in the uk, wether he will kindly

extend the expiry date... just have a good excuse why thou...worth a try .. wish i had known 3yrs ago
save you money, time and form filling
http://www.britishembassy.gov.uk/ser...=1034348124299
1.3 - Validity of entry clearance and leave to enter
The validity of entry clearance and leave to enter are normally the same. There are two significant dates on the entry clearance, the 'effective' date and the 'expiry' date. In most cases, the 'effective' date will be the date on which the entry clearance is issued. However, some applicants may not intend or be able to travel to the UK immediately following their application and Entry Clearance Officers have discretion to defer the 'effective' date for up to three months after entry clearance has been authorised. Therefore, to prevent unnecessary applications for an extension of stay, Entry Clearance Officers should confirm an applicant’s date of travel. For applicants in long-term categories who are subject to either a qualifying period before applying for settlement, or to a limit on their total length of stay in that category (i.e. work permit holder and their dependants, UK ancestry, au pairs, and working holidaymakers), leave to enter should begin on the date they arrive in the UK (within the three month limit). This will ensure that the maximum leave to which they are entitled is given.
Similarly, an Immigration Officer has authority to vary or extend leave where permitted, on arrival in the UK."