I have to ask--does it reach the level of drawing blood? Are there permanent marks left, at all? Has he ever had longer term health risks associated with the behavior?
Not that a lack of any of the above would leave you without basis for attempting to discourage the behavior. But I have to ask...do you just want him to stop because you, yourself, don't like seeing it? Is that the real reason?
Personally, I think he has a right to control his symptoms in whatever ways work for him. I understand that this technique could be particularly disquieting to you and others in the vicinity. But if there aren't any other risks, near or long-term...is it really preferable for him to instead leave his symptoms uncontrolled. Just food for thought.
If you were to describe the symptoms he's trying to control, at least somewhat, it might be easier to suggest something that could be substituted, if he happens to be willing. But if his best friend already tried in vain to dissuade him, it sounds as though he's likely to resist your efforts as well.
I think you might consider counting your blessings. If he's found a way to control his symptoms that works for him--you're more fortunate than many supporters/sufferers with PTSD, after all.
There are some medications that might help...a number of SSRI antidepressants help with OCD symptoms, which is what this behavior may fall into. But it would help to have some more information--ie what are the symptoms he's trying to control...how does the behavior help control the symptoms, in his opinion? Is it conscious, or unconscious?
There are non-pharmaceutical supplements which help reduce stress levels, but I'm not a licensed practitioner, and don't know the forum rules on such recommendations, right off. You might run a search on "supplements, reduce stress, reduce cortisol, sedation", for example.
That's about as much as I've got. Glad you're trying to get the best help for your husband, and welcome to the forum. But you might try the section for supporters, for yourself and your own stress and related situations. You may well find better advice on how to look at the situation altogether from someone in a similar position.
Best wishes.