Recovering from a traumatic incident, whether it was a one time incident, or as a result of military service, is unlike anything you as the friend or family member of this person are probably familiar with, if you have never experienced a truly traumatic event in your own life. So, the most important thing you can offer is patience.
You may want to try and get the person to talk about what happened, but you must be patient and let them decide when and how they are ready to talk about it. Not everyone shares their feelings the same way, and particularly with military or law enforcement personnel, sharing is very, very hard for them. They are trained to keep their emotions in check as they perform their duties, and "wearing their heart on their sleeve" is not something that is encouraged for the safety of the person, the team and/or the unit. They cannot simply come home to you and instantly open up. Trying to make them talk will only make the traumatized person feel more isolated than they already do. What they experienced is literally locked inside their head, and it takes time and patience for them to feel safe again in order to let anyone else know about what they went through. It is the body's way of coping. It sounds bizarre, but anytime there is an injury, whether physical or emotional, the body shuts down certain aspects of itself until it can figure out how to deal with what happened. This happens in various degrees of "shutdown" such as shock, coma and PTSD.
So, the greatest gift you can give to your friend or loved one suffering from PTSD is to be patient. Say to them, "I love you, and I'm here for you." And don't ask any questions. At all. Offer them a hug, or offer to hold their hand, and don't take it personally if they say no. It does not mean they don't love you. It just means they need some space.
Remember that PTSD sufferers are recovering from something that was very emotionally terrifying and traumatizing for them, so don't push. Women especially are notorious for wanting to take care of our loved ones, but now is not the time. The best thing you can do is to not take it personally and become informed. This is not anyone's fault. Especially with children who are used to that person acting more loving toward them than they are now, let them know their Mommy or Daddy loves them and that this is not their fault and that Mommy or Daddy is not angry at them.
Study about PTSD to help you understand what the person is going through, pray, and do whatever helps you in your efforts to be patient. It will not be easy, but it will be worth it, and when things are better, your friend or family member will remember that you were the one person who did not push them to open up at a time when they were simply not ready to be pushed.
If you are patient, you may just be the one person they decide it is safe enough to open up to. IF that happens, let it be at THEIR pace, and at THEIR choosing. Just listen. As much as you may want to talk, don't. Be quiet and just be there.