Omg just ignore her! Ignorant people are usually the loudest talkers. Breastmilk is never dirty and I find it very offensive when she says that. Colostrum is rich in immunoglobulins, which protect infants from viruses and infections.
I haven't encountered the exact same situation but I've heard lots of ignorant statements from people who have never lived through or experienced certain things but they act like they are the premier experts.
Just ignore her. No point getting angry over ignorance. If you can't ignore her, let her read this article:
http://www.llli.org/llleaderweb/lv/lvdecjan05p123.html