Final answer:
If stressed by a large fish during a night dive, your buddy should remain calm, communicate with diving hand signals, manage their breathing, and ensure buoyancy control. Ascend safely if necessary.
Step-by-step explanation:
If your buddy is startled and stressed by a large fish during a night dive, it is important to remain calm and use established diving hand signals to communicate. Your buddy should breathe slowly and deeply to help manage stress and avoid panic. Reestablishing and maintaining buoyancy control is crucial, as is sticking close to you for mutual support. If necessary, slowly ascend to a safer depth or return to the surface following proper safety procedures to avoid the risks associated with pressurized gases in the body, ensuring to equalize pressure and control buoyancy. Remember, unexpected encounters can happen, and preparation, training, and calmly executing practiced procedures are key to handling such situations.