Suck it Up. I am sorry to say this Mamma bear but it is the only solution. My brother went through this we hated and I mean LOATHED his girlfriend. When he went away to work she played all kinds of games and showed her true colors. She truly is the absolute horrid of horrid. Unfortunately he loved her and in order to keep the peace we kept our mouths shut. At twenty years old you can't decide for him any more. Life throws lessons at us that we must learn and this is his.
All you can do is smile politely and say exactly this "I may not like this relationship but I love you so I support your decision because it is your life" He will respect that and maybe it may mend a fence or two. Remember he's only 20 he'll move on eventually. If however he doesn't remember it is his life. You can't live it for him. (And trust me we children often wish our parents could) just be his mom and let him know if he needs anything no matter what you will be there.
ANSWER THIS QUESTION
Butt out of his love-life! You certainly seem to have taken a disliking to this girl (and, believe me, both of them ARE picking up on that), so you may not be all that objective a judge of her. Even if you are dead right about her, your son will not only not appreciate your well-intentioned interference, but could really resent you for it.
Life is full of mistakes and trial-and-error - that's how we learn and grow. If you don't let your son live his own life (including making mistakes) you'll turn him into nothing more than a pathetic excuse of a man - a momma's boy in an adult body. You love him, and be supportive of him, and you do NOT convey any "I told you so" attitude if things between him and his girlfriend don't work out.
I know of what I speak. My mother had your reaction (except for the "daddy's money" part) when I began dating my wife. On a good day, they are cold and dismissive of each other - on a bad day, the mutual hatred could melt steel. It has put a huge strain on not only my marriage (NO man wants to get caught in a feud between the two women in his life whom he is supposed to love), but also on my relationship with my parents.
ANSWER THIS QUESTION