Dianne,
I am going to give you a different perspective, and one that I think is extremely important, but it is because of my own experience.
When I was about to make a big mistake to marry a man that was terribly bad for me several years ago - I was about 24 at the time - my mother and sister expressed their concerns in a couple of different ways on a couple of different occasions. I had been involved with this guy for four years, and lived with him for 3.
One way was loving, supportive, but "please be sure you know what you're doing".
The other was a series of subtle remarks over the four years I spent with this guy whenever a situation would arise, about how they disapproved and frowned upon his reactions and personality. As time went on I shared less and less details about my relationship to him with my family because of these reactions.
You are probably wondering what the outcome was: I was terribly hurt, resentful and felt betrayed by my family. Even though they had the best intentions, I felt completely unsupported and angry. I felt that my family didn't trust me to make adult decisions, and felt infantalized in their presence. I was resentful that they thought they knew what was best for me, because I felt that only I did. I was in a place where somehow this unhealthy person was fullfilling my needs, and I felt like no one understood that. I was in a great deal of pain - had lost my father suddenly about a year before I met this man - but everyone thought I was just blinded by love and having fun because I never showed the pain to anyone. To an extent, they were right - because I didn't know that I was grieving my dad yet - I didn't go through the proper "stages" until years later in therapy.
My former fiance had movie-star looks and made me feel like the queen of England, even though he was deceitful, unambitious, and not very bright. So my family expected me to be appreciative of their concern - instead I distanced myself and drew closer to those who were supportive of my unhealthy person. I can tell you that it wouldn't have mattered who or what was said to me, this was the man for me, and nothing was going to stop me - and anyone who saw it differently didn't really care about me because they couldn't understand that he is what I needed at that moment. Even though they meant well, even the slightest lack of support of this relationship from my family made me feel bad about myself because they questioned my decision making ability. So I kept far away from them! I felt so betrayed from my family, and when things did start to get bad with the guy, I felt I couldn't confide in my family - because they would just say "I told you" or "see, you should get rid of him" instead of just listening or being supportive.
Luckily, I discovered on my own about 5 months before the wedding that this man had been unfaithful to me for 6 months while we were living together. I dragged him to couples therapy to try and save the relationship and soon after became strong enough to realize he wasn't what I really needed/wanted. We never married, and I moved out of the area where he was from after a month.
Dianne, I am about to enter the world of parenthood, 5 months pregnant at age 36 and very happily married to a healthy man. I probably would give you different advice in another 20 years, like some of the posters here whom have had advice with children. However, I think it is still soooooo important to remember the "child's" perspective. And I can honestly say, even now years later, I am married to a wonderful man, have learned many life lessons, and looking back, I would still have wanted my family to have acted differently back then. I still feel they were wrong.
Your son is still very young, and here is the reality - he is probably going to make this mistake, and learn the hard way. Please, be very careful about giving him advice he does not want to hear. If you feel confident that he is intelligent, you have raised him well, etc. - there is not much else you can do except hope for the best that he will figure it out sooner than later. In truth, intelligence has nothing to do with matters of the heart and deep psychological needs - no matter how unhealthy they may appear on the outside. I fear that if you show the slightest bit of disapproval you will push him away, and what he will really need, when things get tough for him - is to know that he can call you up when this jerk girl has hurt him, and just listen, and not tell him, "see, I told you". He will probably go through a few years of this frustrating stuff as you silently watch her do these terrible things - and it will be tortureous. But I don't think I've ever heard of a story of an adult child coming back to an unsupportive family after a divorce and saying, "MOTHER, why didn't you TELL me I was making a huge mistake? Why didn't you TELL me this person was a terrible narcissist?" On the contrary - I would have really appreciated my family if they would have supported me no matter what. I guarantee, would have come back to my mom and said, "Gee, mom, I sure was stupid about [former fiance]. But thanks for trusting me and letting me figure it out. You were there when I needed you."
So, going against the grain of everyone else here - I say, if you really love him, show your support that you TRUST him, that if he believes this is the right girl for him, that he has your blessing. I think, despite everyone's intention here of saying "deliver the message in a loving manner", it doesn't matter HOW you deliver the message - the message still is - I DON'T APPROVE. This is going to hurt his self-esteem, no matter how old he is.
Even if ever fiber of your being tells you differently - there may be something he is getting from her that you don't see. And lets not forget - he may have his own demons that she is accepting as well. He will figure it out, and unfortunately, it may not be until after they have been married for a while. But it is his life to lead, ultimately - and he needs his family to fall back on and for support.
I really hope you will hear this. I know it would have made a real difference to me had my mother heard me. I felt voiceless. She didn't HEAR me. Please LISTEN to your son. If you really care - sit down with him and instead of telling him what you don't like, ASK HIM what the things are about her that he loves. Maybe you can learn to identify what those things are, so you can better understand why he needs to be with her. You might even try to find common ground with HER based on those things - he would really appreciate it I'm sure.