I had a simialr problem with my son, who held me accountable for things I never did, thought, said of felt even remotely. He has so far (to my knowledge) not leaned on a woman as your son has to you and his "friend". I agree with "tough love" necessity. It will not be easy. If I were his father I would tell him that hee needs to "man-up" and that does NOT mean worming it out with a girl friend. Get a job and be somebody contributing and supportive instead of a parasite. His girl friend will get it at some point and either try to change him or leave him. Women aren't stupid, indeed their heart is a superior gift very much like God and capable of great and amazing good. Either the girlfriend or his honoring or violating it will have life long ramificattions. Love without respect is presumption. Repect without love is fear. He and she need balance of each and God to make it work out in their favor.
Aside from what I have written, you have found good support and prudent counsel here.
I had similar issue of confronting my youngest brother years ago regarding his fiance at that point - among other things their living together. He argued with me. As it turned out his best friend, a married protestant man silently dated the woman, which resulted in divorce. My brother broke up with the girlfriend at great expense and pain. The great story of God's redemption is that he eventually was sorry for his sins and God heard him. He was very JOYFULLY married November 10 to the woman he always wanted to date in high school. While lots of weddings are inside church buildings, this small wedding was profound for the presence of God in it. They are three in that marriage and it is truly one of the best weddings I have ever seen in my life. I hope the same for your son. Confronting my brother was tough.
In another case, I confronted my cousin, who was in seminary. I told him of a dream I had of him. It was not what he wanted to hear. Things cooled between us for probably two months. The short of that story is that he remained pure and became a wonderful priest and military chaplain in the USAF. The woman, who he was becoming serious about, professed vows in Mother Theresa of Calccutta's sisters of charity in San Francisco. Her brother became an extraordinary priest, who many think will one day be a bishop. Tough love is not always easy, and one has to be CHERITABLE and FIRM simultaneously as Christ was with some people - even His own Apostles huh? However, it is necessary to authentically love instead of conceeeding badly to the loss of reality for all concerned. Your position with prayer to God will affect even those you may not be aware are impacted.