I can't say specifically about war-related PTSD but in general it presents itself as flashbacks and nightmares, and severe anxiety and panic attacks, triggered by anything that reminds them of the traumatic event. So that might be someone talking about it, a tv programme, a photo, a thought. A sound, sight, smell of anything at all that they connect with the event.
They will probably find it impossible to talk about their experience as by doing so would trigger an attack so the way to cope will be to block it out completely in the hope that the bad thoughts will go away. However this usually doesn't work and untreated it can stay with them for years, even their whole life.
They may become depressed or try to block it out with drink or drugs. Relationships with others may become strained as they struggle to deal with their thoughts. They might not be able to sleep for fear of nightmares.
Treatment relies on the individual firstly recognising they have a problem and it is then addressed by getting them to actually confront their fear of the event. They might be asked to write down everything they can remember about it, if talking is too difficult. Slowly they would be asked to 're-live' it by talking about it and discussing all the things they find hard to deal with. This would be done with a trained counsellor and is usually successful. They would be given techniques to use when a panic attack is triggered so that it is less debilitating. The more they face it the less traumatic it becomes until eventually they can think about it without the associated panic.
As far as what you can do to help goes, just be around to talk and listen. Encourage them to seek help if it seems that they are struggling and tell them that it is very common - not only for war survivors but victims of car accidents, those who have seen others hurt, and many other traumatic incidents.
Hope your friend is ok and you can help if they need it.