This must be so horrible for you.
Many years ago when I was an thoughtless opinionated teen, my then boyfriend had a word to me on behalf of his mother. I was truly mortified when my behaviour was brought up and I changed immediately. You are doing the right thing both for yourself and for her personal growth.
When you speak with your son, use *I* statements. "I feel _____ when girlfriend says_______ "
Good luck xx
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.