I didn't see your previous post, but as a close relative of someone who gave 2 children up for adoption I implore you to please NOT give a card.
Regardless of how she is acting on the outside, she is very fragile and vulnerable on the inside and you do not want to tip the scale ~ remember, until the adoption is finalized she can take the child back. Honestly, this isn't a Hallmark moment, she is giving birth AND giving her child away, both life-altering events, and she will think about this child forever. As pleased as she is with the couple she has chosen, even if it is an open adoption, she will have doubts about her decision and grieve for the loss of this child, possibly the rest of her life. As you said, it was a hard decision for her to make, and she may require counseling to deal with it. You truly cannot know the emotions she is feeling. Any reference to what she did, regardless of how you praise her for her decision, or how good she is feeling about it, may feel like a knife going through her heart. And the fact that she is bi-polar can only intensify those feelings.
If you are a good friend of hers wait until she herself brings the subject up, at which time you can then relay your admiration and respect for her. If you are not a good friend I wouldn't say anything directly to her, but instead tell your BFF and she, if the right moment presents itself, can tell her then.
I have the utmost respect for women who think of their child first and place them for adoption, but having seen the turmoil they can go through for many years afterwards, I will not do anything but be a shoulder to lean on while they cry or an ear to listen. Placing a child for adoption is something that can traumatize a person in ways you've never thought of, please be considerate and respectful of her (I know you are.)
God bless her and women like her❤