Likewise, regret doesn't have to be big, scary or demeaning towards another's experience. A) read the chapter in Lifegivers concerning birth parent regret (chapter 8) and b) read
this post and
this post concerning how I, myself, and other birth parents are viewing the topic at hand.
I'm sorry your son's birth mother had a negative experience but regret doesn't have to be negative. It's about how it's handled, how it's processed, how it's dealt with (not avoided or blown up) and how it's presented.