With talking to veg*ns, I try to be very respectful of their opinions even if I disagree. The truth is that a well-planned vegan diet can be healthy for some people. There are people who cannot convert ALA to DHA (such as children, though usually the mother can, so as long as they are breastfed until they can, perhaps it's OK?) and people who cannot convert beta carotene into Vitamin A who should not be vegan, but beyond that, there are ways to get the right nutrients as a vegan. In my opinion, it is easier as a vegetarian and even easier with vegetarian + fish to get the right nutrients, but if somebody wants to be vegan, there are good ways to do that.
When talking about nutrition, there are a lot of ways we could all improve our nutrition through foods that do not involve animals. I would be very careful of coming across as critical, but you could share your love of ferments: water kefir, fermented fruits and vegetables, condiments, etc. These provide Vitamin B12 and help make people more able to absorb other nutrients. Seaweed provides iodine and other micronutrients. Pumpkin seeds are a pretty good source of zinc. Coconut oil, olive oil, flax oil, and chia seeds are great fats for vegans, and add butter and ghee if they are only vegetarian, and couldn't we all eat more vegetables? If the situation is right, talk about your own feelings about soy or how miso and tempeh are better than tofu and soy milk or if you have problems with gluten or grains or feel better when you keep your blood sugar in check.
I certainly wouldn't bring it up with her as an accusation about how her diet is going to hurt her baby, but if she is your friend, you can certainly share some of the things that you find interesting. Don't make it about changing her, just about what you are learning and thinking about. She can and will make the best decisions she can for her baby. It's hard to be in a position where you're caught knowing something that could help somebody, but you know they wouldn't be receptive to the information.