For her to be able to apply for ILR later she needs to be resident in the UK for 2 years, normally the visa is valid for 27 months to allow any slight delay and gain the 24 months in the UK.
Check the dates on her visa to see when the latest date she could travel. If you still want to delay her arrival then you could contact the embassy explaining her situation to see if the can change the dates on her visa.
