should I tell my husbands girlfriend about his porn addiction?

I am some what confused about the situation since there isn't much detail but I guess I would not.   If he is still your husband and he is with you then he shouldn't have a girlfriend.   She will find out eventually but it might make things worse coming from you.  I once had a boyfriend for seven years who had his own addictions.  He cheated on me with a woman and left all the traces for me to find out.  Why? I dont know he was not right.  Anyways, I called her up and told her all about him and she became rebellious and did not listen to me, later he lied about going on a business trip and got her pregnant came on my birthday to tell me we were going to break up and I asked why and he shared about his unfaithfulness.  She regrets being married to him and they since then had many problems but nothing is easy in life it is all a challenge it is how we handle the challenges and ourselves when we are going through them that matters.  I hope you find strength to do what ever you feel called to do.  I pray God will direct you and that you can find the peace that surpaces all understanding that only God can give you.  PM me if you need to talk some more.