If his 'phone has been lost or stolen?
Even then, however, it should be down to the prosecution to make a case for disclosure in front of a judge, rather than the police to seize mobiles in every case.
Even if I were accused of rape, one of the nastiest offences going, I would not demand my accuser offer up that amount of personal and confidential information - not to mention her 'phone / address book, photo album, diary and calendar - unless I had a very good reason and it was unavoidable.
I appreciate the seriousness of rape and the difficulty of securing convictions, but this is an additional tool for guilty people to escape justice. Not only that but, in a nation where low-level crime is routinely ignored and our police are overstretched, where are the resources for checking through and analysing the 'phone's content in sufficient detail? Without interviewing both parties extensively, how do the police know that "u ok hun?" does not mean "Are you recovering from the trauma of being raped by me last night?"