Flood himself, with Michael Jackson and others, has done much to bring this sort of systems thinking into the world of Information Systems, though it remains quite technocratic - and requirements engineers still have a lot of work to do to bridge the gap between 'users' and 'systems'. But Friedman is both setting an agenda and carving out a territory. The question hinges on whether situations involving humans are largely repeatable and predictable, or fundamentally different every time, personal and unique. For me, the distinction is nowhere near as sharp as for the authors. For example, the behaviour of every driver in a traffic jam is unique, but the mass, emergent behaviour of the stream of traffic is predictable and can indeed be modelled hydrodynamically with good results.
Several authors reflect on the methodology itself.
Where both are fully involved, you have Action Research in all its forms - seemingly at least 40 of them. Conversely, where the client calls for the project, and both consultant and client are fully involved, you have process consulting. All these modes, and others that Schein identifies, make excellent sense in disciplines such as business process modelling and requirements engineering, not to mention conventional and social research. This is a textbook which can be dipped into, searched systematically, or mined as a source of further reading. Many Ph.D. theses will no doubt be planned through its calm pages.