Your friend needn't worry about where she is staying, the council will probably just deem her friend's house as her main residency, as long as she has some kind of proof of address there. She would then have her children placed locally to that address (depending on spaces). If she subsequently moves somewhere else in the borough she always has the option on an in-year transfer.
As RumourMill says, pupil services are her best bet. They are very busy at this time of year though! It might be prudent to wait till midway through September before contacting then as there is always a bit of a churn at the beginning of term.