I don't think the cooking necessarily means she's hiding it, maybe her partner just feels less icky about eating cooked blood.
I don't know, it might be because I'm a vegetarian, but I don't actually see the problem, in principle, with eating stuff like menstrual blood and placenta, since nobody was hurt, and otherwise it goes to waste. So if you're going to eat meat, I can see the sense in eating meat that's already there, if I'm making sense. It kind of prevents a waste.
I could see feeding it to other animals, too, and I guess burying it for it to feed plants works as well. It all seems better than just throwing it away.
This being said, I could never cook with menstrual blood, be it my own or someone else. If someone was serving their placenta though, vegetarian or not, I'd probably be too curious to say no, and I'd at least give it a try.
Bottom line though, you should definitely be open front about it, so no serving stuff without telling people what it is, and no inviting people and serving stuff once they're there and "trapped" without warning them beforehand.