Adultery is only a crime under the UCMJ and is perfectly legal for a civilian. The spouse that your husband cheated with will not be legally punished for this. The Marines have no jurisdiction and it's not illegal for civilians.
If she is not in the military--she can't be punished by the military because she is not subject to the UCMJ as is your husband.
I am licensed only in the Commonwealth of Virginia, I try to provide sound guidance but I am not providing legal advice, you are urged to contact an attorney in your home state for specific guidance and legal advice. We do not have an attorney-client relationship and I am under no obligation to respond to subsequent emails or phone calls. I wish you the best of luck.
As noted, civilians are not prosecuted.
One thing your husband could try, but it might be too late, is an appeal of his NJP. (Navy or Marine?)