Failing a promotion board is a disappointing experience for a Soldier. No matter what you do, try to focus on the way forward. This is a temporary setback. It can be overcome with hard work.
AR 600-8-19 paragraph 3-47e states:
e. 5. REMARKS. Each voting member will for the purpose of counseling, comment on specific item(s) in which the Soldier appears noncompetitive and/or weak in this section.
Include these comments. Make sure the Soldier knows how he can improve. It would also be wise to include the next date that the Soldier can appear before the board.