Ok, thanks for all the smug replies!
I was posting, not so I could have a reason to tell this woman I felt sorry for her son - I am not that insensitive. Not sure where you got that idea from!
The point is that this woman is so responsive in every other area of her parenting and someone she trusts has told her that it is important to do CC with a toddler. She is therefore doing it, even though her heart is breaking. She has no actual need to do it - there are other options. And because I know how she actually wants to parent her children, I know that she is going to feel desperately guilty about this if/when she finds out that CC can be harmful.
In addition, it is not me who is linking it with CC, it is this woman's friend who has 'taught' her how to do it based on this assumption that you just ignore your child for so long. Even those of you who are ok with doing CC are disagreeing with that interpretation of it.
And saying CC isn't harmful IMO is different from saying it categorically isn't harmful and that that has been proven (particularly when they've shown that in fact it has, but that's another debate!).
The point is, this woman has only heard one side of the story and is making a decision I think she could bitterly regret one day.
Anyway, I ended up writing her a long letter and leaving it at her house this morning with my copy of The Science of Parenting by Margot Sunderland and, very tactfully, suggesting she may want to read it before she continues just in case there is information she hasn't yet got in there.
She texte me this afternoon thanking me profusely. She hasn't said what she's going to do, but she clearly doesn't hate me, thank goodness!