Maybe the friendship route for female Wardens is decided by:
1. "Defeating" Flemeth and giving Morrigan the grimoire
2. Persuading Alistair or Loghain to do the dark ritual
Not to mention choosing not to stab her in Witch Hunt
1 - The two wardens are alone in Ferelden remaining to stop the blight. Their companion knows how the blight is ended, but does not say. One out of two will die. The two may be friends or lovers. They are not warned.
2 - The alternative is that they are one among hundreds, with the seniors lining up to take the final blow first.
If I was told I had to try to defend one state of affairs, I'd try to defend the latter.
Personally, I can see why she didn't want to tell you about the Dark Ritual, particularly if you romance really anyone in the game. Think about it:
1: The fact that a Warden has to die is a terrible fact, and for 3/4 of the game, at least, it seems the PC and Alistair are the only two left.
2: If the Warden is male, there's no promise that they've formed a relationship. If the PC is female, she CLEARLY doesn't like Alistair and he doesn't like her. If the Warden IS in a romantic relationship with her, then how is she supposed to say that in order for your character to have more than a 50-50 chance of survival, you need to conceive a child RIGHT before the battle, whether you're ready to cope with that fact or not.
3: If the PC is female, she has to ask you to convince either Alistair or Loghain to sleep with her. That's just awkward, there's no other way to put it
I will admit that she's harsh, she speaks her mind and she keeps her secrets and she's very pragmatic but I honestly can't blame her. Not everyone is an open book, not everyone is going to be easy to like and some people are not going to be working to save every kitten stuck in a tree. That doesn't make her a bad person, that makes her a realist. I admire that