You've gotten a lot of thoughtful responses already, with moms who have made some good points, so I won't bother repeating them. I can understand how you would be upset (as much as someone who doesn't have fertility problems could possibly understand, which is insufficient, I'm sure), but maybe this will help put it in perspective...
If the friend's daughter who is giving her baby up for adoption were 25 and living on her own, instead of 16 and living with her mom (your friend), would you be nearly this upset? It sounds like you're upset because you feel your friend should have considered you or at least should explain why she didn't consider you, but as others have said, it's not your friend's decision, it's her daughter's decision. It sounds like maybe because the girl is 16, you're assuming the mother should/could/would have more influence on the decision, but that may or may not be the case.
As far as whether to say anything...I guess it depends on how close you are. I would probably not say anything. If I did choose to say something, I don't think I would lay all my angry and resentful feelings on her. I might instead say something like, "I can imagine why you may not have wanted to consider us to adopt your grandchild, but we are looking to adopt again. If you hear of anyone else looking for adoptive parents, I'd be grateful if you passed them our contact information." (You never know, during all of this, they may cross paths with another pregnant teen who has not yet chosen an adoptive family.)
Saying something like that isn't too confrontational. It creates an opportunity for a more in depth conversation about why she didn't consider you, without forcing the conversation to take place. If you were to just tell her how hurt you are, I think it may make her feel as though she must defend their decision. It may also make her angry at you for making it "all about you"...after all, their situation is probably just as painful to them as yours is to you.