Tell your liberal friend to read Muir's 'The Caliphate, Its Rise, Decline and Fall.
http://answering-islam.org.uk/Books/Muir/Caliphate/index.htm
And on my bio page, there are links to the works of Edward, Canon Sell. One of them is the biography of mohammad published in 1913. pdf download.
OT but great pics on bio page ? Whereabouts are they ?