Many who have PTSD do escape through the use of drugs. Another way to push people away and isolate is to say "you do not understand blah blah blah."

How do you stay friends with someone with PTSD? Just be a friend. Try to keep healthy boundaries. Don't rescue them from their problems but honestly guide and point the direction to reality. Don't make excuses for their behavior. PTSD doesn't give one a license to hurt others, even verbally. Drug abuse doesn't excuse behavior either.

Just my two cents.