The femtosecond dynamics of NaI two-body (NaI + h nu --> NaI+ + e(-)) and three-body (NaI + h nu --> Na+ + I + e(-)) break-up is investigated by quantum mechanical simulations. A first femtosecond laser pulse creates an excited molecular wave packet, and an additional ultrafast photoionization step monitors its subsequent time evolution, The theoretical results are compared with available experimental data. The branching ratio between these two competing fragmentation channels is extracted and an experimental scenario is proposed to control the outcome of this reactive half-collision,