As a parent, I would probably never watch an UNRATED version of any movie for the first time with my family. That is just who I am.
That mother can only blame herself for watching an UNRATED movie and shocked when she sees something she didn't expect. Ratings are useful and if she was really that sensitive, then she should use several websites that review movie content, screen PG-13, R and Unrated DVDs, etc.
There are plenty of tools for parents to use and nothing beats good old fashion screenings.
Ratings and pre-screening are great tools. I own unrated material, etc. But I use the tools available and if I am still unsure, then I prescreen. Pisses me off when I hear about people complaining that someone else isn't protecting their family from material they find offensive, regardless of nudity, profanity, violence, etc. It's not like it was on public television. And even if it was, they should update their hardware to take advantage of ratings controlled blocking, etc.
But in this case, it was a DVD. So it's completely her fault. It was listed as Unrated and so she should have found the rated version as a guide, used an online tool, or watched it first. No one forced her to watch it with her [censored].