Learning some breathing techniques can often help. I face the person with anxiety arms length apart. Tell them you will do it with them. Hold their hands and together breath in deep and exhale really slowly, three or four times together. Repeat it again. It gives them reasurence that it hopefully works. Have some relaxing music on in background. Another way is swimming which I have said on here before, has a really good evidence base. Just being in the water can be relaxing. Same riding a bike. To he most obvious one is reasurence. Tell them they are safe, your here for them, they have friends and be available to them wherever possible. People who are anxious and experiencing PTSD need to feel safe first and foremost. Tell them it's trial and error until you fall back on the one that works. I'll give you one more which I know has worked for a log of people. Knitting. It's apparently very very relaxing and did you know it was utilised by many of the men returning from combat. It was done in groups, people socialised, took their mind of things. Don't laugh at the knitting one my friend, it has worked for a lot of people and you know what, if it works for you, crack on. So reassurence and exploring what works for the person. If things don't work, leave it and try another thing but fundamentally its about comfort, safety and re assurence. Good luck to you and your friend.