If a member leaves midway through a project, the first step is to reassess and briefly redistribute their responsibilities to capable colleagues. Also, to prevent disruptions, I would have the exiting person thoroughly document their procedures and, if possible, be involved in a brief transition. Meanwhile, holding a short team meeting to reassign duties is helpful in maintaining project momentum.
If time permits, some limited cross-training in advance can smooth these transitions. Having a backup ready ensures continuity.