It seems to me that the woman could sue the girlfriend for defamation of character.

Re: Carol Ann--You can't sue because someone says nasty things about you or there would be lawsuits all over the place. You have to prove actual economic damages or your case probably would be thrown out. This ex doesn't care that his girlfriend is making these statements and may well be getting some pleasure out of the discomfort of his ex-wife. In the end, it won't do anybody any good but some people go for short term "benefit" with no consideration of what damage it is doing to his son by putting him in the middle or the effect on his ex-wife's willingness to deal with him when child custody issues come up. The girlfriend might actually be trying to keep the boy from visiting by causing trouble for his mother because she either wants the boyfriend to herself or doesn't want a kid around at all.

http://www.videojug.com/interview/defamation-defenses-2

I am having a hard time with the first letter. I think the writer isn't being completely truthful about everything. The girlfriend is putting the son is a bad position and trying to cause problems. While the writer needs the girlfriend to back off, she says ex is a great father. How can that be? It's impossible to be a great dad if you are allowing your son to be around such a manipulative, hateful woman. I think both women here are at fault and the writer is jealous that ex has a new girlfriend and has a hand in all the defamation going around.