God forgives all sins. So there’s no need to worry about that. The question might be whether this was even a sin. Your husband may have not had intended to kill the person in this encounter and instead acted if self-defense. For something to be sinful, one has to have intent to commit the act and have full knowledge that the act is sinful and then, has to actually carry it out.
In this instance, let’s say your husband had every intention of finding an enemy and killing them. That indeed would be sinful. A good confession would be appropriate (and healthy) for him to share much of the horror of war with God and to ask for forgiveness in his part in the war. War may be necessary and one might be following orders, but we are bound not to take the lives of others. We may not have found a way to avoid war and violence, but that is our problem. God calls us to peace. That said, God also forgives us, for in our human weakness, we often succumb to evil because we don’t know any other way to solve the divisions we have as countries and as people.