Why should doubt arise in your mind? Your friend wants to marriage the 1st wife or x or y or z while his legally wedded wife is living. This bug should be removed from his mind. No divorce, no marriage.
He wants? What is the value of his want? Does his 1st wife also want and is the second wife ready for divorce? Remember in giving divorce, aliminony and maintenance may make him pauper.
I think living with any woman without getting married (e.g. live-in relationship) is not adultery.