(Forgot to post this recently)
The Oxford English Dictionary (the big one, on CD-ROM) says this:
[The name of the litigants in the case of McKenzie v. McKenzie, in which the English Court of Appeal ruled that any party in a trial is entitled to non-professional assistance in court.]
A person who attends a trial as a non-professional helper or adviser to a litigant without other legal representation. Also attrib. in McKenzie friend, man, etc.
[1970 All Eng. Law Rep. III. 1034 McKenzie v McKenzie+. Any person, whether he be a professional man or not, may attend a trial as a friend of either party, may take notes, and may quietly make suggestions and give advice to that party.] 1973 Civil Liberty July 4/1 Unable to afford lawyer’s fees Miss Fogarty enlisted the services of a McKenzie. 1974 Observer 15 Sept. 22/3 A defendant may have a friend beside him+to give moral support, take notes, and generally give a helping hand. Known as McKenzie men+, these helpers can be either lay or legally qualified people. 1979 Internat. Jrnl. Sociol. of Law Feb. 115 The Centre should adopt a more aggressive role by developing the legal competence of clients+by encouraging the use of MacKenzie men, and by demystifying the law. 1982 J. Pritchard Penguin Guide to Law lix. 871 Every DIY litigant should+exercise his right to have a McKenzie man. 1990 Daily Tel. 24 July 2/5 Mr Dave Nellist, MP for Coventry, said he intended to appear before Coventry magistrates as a McKenzie friend.