Oddly, although magistrates have a document (consisting of over 450 pages) giving them guidelines for sentencing almost every offence which can come before them, it doesn't include the offence of wasting police time.
So, working within the framework which the law gives them (i.e. a fine of up to £2500 and/or a prison sentence of up to 6 months), the magistrates, possibly together with their legal adviser, will have to work it out for themselves.
Here are a few examples of the penalties that have been handed down by other courts:
https://www.criminallawandjustice.co.uk/law-digest/Magistrates%E2%80%99-court-wasting-police-time-1
http://www.iomtoday.co.im/article.cfm?id=32775&headline=Couple%20lied%20about%20assault%20to%20the%20police&sectionIs=news&searchyear=2017
http://www.bbc.co.uk/news/uk-northern-ireland-37187121
http://www.bbc.co.uk/news/uk-scotland-tayside-central-20254822
http://www.loughboroughecho.net/news/local-news/man-sentenced-16-weeks-prison-5806141
My guess is that the magistrates will impose the maximum sentence available to them (after a one third reduction for an early guilty plea), which is one of 4 MONTHS imprisonment. However the actual time your friend would be 'banged up' would be 2 months (or possibly
slightly less if he was eligible for tagging)