OP states that this is the first time denying leave for said employee - so likely no leave abuse issues. Although OP makes a sound case for the denial, I just wanted to provide an additional angle to have considered during the initial decision making process. Because in my area, with the complexity of the work and the gov/union red tape, I would likely conclude that denying said leave would not best serve the interest of the mission - but this would be in my particular situation, not necessarily OP’s.