[Always *]

Well, just remember, if you tell him, you'll probably effectively end the relationship. I'm not saying he doesn't deserve to know, he should, but definitely talk to her about it first and make sure she is ready to face the relationship ending and give her the chance to say she wants to be the one to confess. It is her relationship after all.
And yeah, like everyone else has been saying, you should get her to go to the doctor. That might want to be something you do before telling the friend she's pregnant, because if it isn't yours there is really no point in being like "oh yeah, she's also pregnant with my kid" if it isn't yours cause that'll probably just piss him off even more and you might as well do all you can to minimize the damage. If it is yours you 100% have to tell him. And to be certain, you should do a paternity test.