L. John Fry is also known as the "father" of methane production his first working plant in South Africa dates back to 1956, his designs still are emulated in the field today. The Mother Earth News built several designs back in the 70's and early 80's and have many articles on the subject.
Fry's system mainly consisted of a large anaerobic digester, 2 converted diesel engines for electric generation, a good storage design and some scrubber systems.
Journey to forever has one of his publications available on the web: http://journeytoforever.org/biofuel_library/MethaneDigesters/MDToC.html
A must read for anyone wishing to get into this.. the University of Colorado also did extensive research into the subject at one time.
It really depends on the scale of production your friend wishes to get into, and what raw materials he has to use.
For a digester to properly convert any waste materials the temperature should be held between 80 and 100 F this is usually done by burning off part of the methane produced to heat a water loop. Stirring of the slurry also increases production.
Safety is a VERY important factor in this, as a mixture of between 4% and 25% methane to air is explosive. You must insure there is no available oxygen content in your storage and piping systems, and these systems are considered an explosion hazard.
Usually these systems aren't economically viable unless you run a large farm,and have the waste to deal with. I doubt very seriously you would be legal building a system within a metropolitan area.
Also methane gas has a much less energy potential than any of the other common heating gases.
Getting a system up and running can be difficult and my experiment with a smaller system (200 gal multi-stage built out of 55 gal drums) wasn't all that successful, however I do believe with persistence it can be done.
