Here is a copy of the band descriptors for all 4 sub skills that are tested in the IELTS speaking test.

https://www.teachers.cambridgeesol.org/ts/digitalAssets/114292_IELTS_Speaking_Band_Descriptors.pdf

If it is just the speaking element you are helping your friend prepare for then I wouldn't worry about buying a book for it. There are loads of free example questions/topics for parts 2 and 3 of the test online (just google it) and you can find lots of example answers for them too.

Good luck