Your friend isn’t alone. I also still panic in the water … after 10 years of Tris and open water (ocean) swimming.
Is there something that triggers it for him? Waves? Swell? Chop? Being surrounded by lots of swimmers?
Last time I freaked was when I could not get past the surf zone and couldn’t see land or anyone else… hyperventilated but then eventually calmed myself, reminding that I’m a very capable swimmer and flipped over to float on my back until I breathed normally again.
I know of a couple of people who have never had this issue and they grew up being in the ocean all the time, surfing and doing all kinds of water sports. I learned to swim as an adult, 10 years after my first lesson I got over my fear of the deep water. I love swimming in the open water/ocean (!!) but will occasionally still freak out.
Here’s a vid that gave me some good ideas for swim techniques in different conditions … remind your friend that it may be a mind over matter issue (unless it’s caused by cold water, wetsuit or other factors as other people have noted). I have found that swimming in the open water in all kinds of conditions was the best way I came over my fears. Good luck to your friend.