Relative differential cross sections for the coplanar asymmetric (e,2e) reaction in argon have been measured at 1 keV incident energy in three different kinematics. Depending on the scattering angle, theta a, the chosen kinematics select either ionising collisions belonging to the Bethe ridge ( theta a=20 degrees ) or processes in the intermediate region between the pure dipolar and binary regimes. Experiments are compared with the theoretical predictions of plane-wave, eikonal-wave and distorted-wave impulse approximations. The results show that, independently of the symmetry of the ionised orbital and even at iron recoil momentum as large as 2.6 au, the distorted-wave impulse approximation (DWIA) satisfactorily describes the triple differential cross section, whenever the kinematics complies with the Bethe ridge conditions. Off the Bethe ridge, factorisation of the DWIA cross section into a two-electron collision amplitude times a structural form factor can be a bad approximation.