Personally, I wasn't ready to have a child when I was 19. Or 20. Not emotionally, not financially, not in terms of maturity. And I knew it. So I was fairly careful not to.
I don't know your mate's "plan", how she thinks she has it "all planned out" that having this baby will work just fine. I find myself rather doubting that she has it as worked out as she thinks. Even when both parents completely want to have a child, it still hits like an atomic bomb and nothing is ever the same after. Don't get me wrong, I love my daughter more than I ever thought I'd love another human being, but it made for some huge changes in my life. Not to mention a whole new set of nightmares and a near-constant nagging voice in the back of my head telling me that I'm doing it wrong. (Literal nightmares, that is. If you have terrifying dreams of falling, prepare to have much worse dreams about your child falling.)
But... Not telling her lover that she's trying to get pregnant is both morally wrong and an incredibly bad idea. It's like telling your best friend that you want to show them an awesome view at the top of a building and not mentioning until they reach the top and the roof door is locked behind them that, oh, by the way, they're going to have to walk a high wire up there with no net.
I understand that you feel you can't tell him. And you're right if you suspect that if he hears it from you, he's probably going to feel incredibly betrayed- perhaps even to the point of ending their relationship, and even severing one or both friendships. But you're also going to have a huge, guilty secret hanging over your head if she does get pregnant and it causes them hardship in the future. In short, you've been put in an awful place.
I would plead with your mate to tell her lover that she wants to get pregnant, and urge that they have to work this out between them and decide whether to wait (a few years and a little more financial stability could make this seem a much better idea) or go for it now. I'd ask why she thinks this can't wait until they (and especially he) feel more secure. And to put it bluntly, I'd ask how she'd feel about raising their child without him. If she thinks telling him that she's trying to get pregnant could hurt their relationship, she's being incredibly naive to assume that actually getting pregnant either won't or will make their relationship better.
Which does raise another question in my mind- is getting pregnant really about having a baby for her? Or is she actually afraid of being abandoned and thinks that having a child with a man will make him more committed to staying with her?
In any case, her current course of action is unsupportable. You need to let her know- either so you can change her mind, or so you can make it clear that you're washing your hands of the situation if she continues.