One of my co-workers had PTSD from military service, and his wife was pretty close to a saint. She knew how to calm him down if he'd get agitated about something. None of us understood how his PTSD worked, so we'd just let him take a long walk around the campus until he felt better. She could calm him down with just one phone call.
I think having a relationship with someone diagnosed with PTSD would be very tricky. I don't know if I could do it. They say a lot of people with PTSD are perfectly fine most of the day, then they'll hear or see something that triggers a panic attack. In my co-worker's case, it was any loud, unexpected noise or shouting. His wife made the relationship work, though.