In my expeience, addicts go after overly responsible grown up women in long term relationships. Think about why he tells you about all the details of his new "relationship".
You are worth much better than that - as for the GF, you have not heard her side of the story, plus it is all new and is bound to deteriorate. Especially if GF is needy and whiny - alcoholics don't like to empathize and want spotlight on them and their problems only.