First of all, I think it's awesome that you identified that piece of information as incorrect, and you definitely should tell her. In my Conservation Biology class my professor knew less about certain topics than I did, mostly because I get very obsessive over certain facts. She also told me I was wrong when I answered a question and I knew I was right beyond any doubt so I had to e-mail her and set her straight. Just goes to show that a PhD helps, but it does not mean everything.
If I were you, I would just explain that the belief that "homozygous roan is lethal" was founded on a paper from the 1970s--they thought that it was lethal because there were fewer roan offspring from roan to roan matings than were expected based on what they knew about genetics at the time. This lead them to conclude that it was lethal to the embryo, but it has actually been shown that it's less straightforward than that. There are living horses which have been shown to be homozygous roan.
Don't know if anyone has posted this link yet: http://www.hancockhorses.com/article-roanQHNews.pdf
It gives some good information on the findings by UC Davis and discusses the "forthcoming" paper. I can't find the paper anywhere, is it just not published yet?