While I don't have experience with PTSD, I do deal with being bipolar and it is very hard to control your reactions. It doesn't excuse the behavior, but be aware that it's hard to stop once a person gets going. While it doesn't look like it, the person is trying. It's hard being in emotional or physical pain 100% of time trying to pretend you are fine and it's easy to blow up on people trying to help.

Sometimes anger is a silent plea for help that the person can't express like normal people can.

Before you work with the person again, pull the person aside privately, sit down and explain the rules and what you expect from this person and make an agreement that both of you can keep and understand. Sometimes the "unwritten" rules aren't clear to those who struggle emotionally. Make it clear that you want to work with this person but you can't if the behavior continues as it's not safe for you and everyone else.