A knowledgeable accurate writer does not use the term "former marine " or even "marine" uncapitalized as a proper noun.
With few exceptions, there are no "ex-Marines" or "former Marines," U. S. or British.
As of 13 September 1960, Lee Harvey Oswald (click here) became one of those exceptions.
Please accept this correction as a well-meant improvement to your writing for the honor and history of the United States Marine Corps.
I know the rules about Marines, but I figured LHO was an exception to the rule.