What would probably happen....

Realistically what I think would've happened is Lu would've gone to prison for kidnapping and I also think that the mother would be thoroughly investigated by CPS and probably charged as well. If the police were to check the cameras at the hospital they would see anico arriving with Lu and then telling her to leave as he takes the baby, to me that would be enough to possibly charge him as an accomplice. So possible jail time there too? Maybe just a fine idk.

In the movie though it seemed like the detective wasn't going to be so hard on her and really believed her side and probably was going to help her get a deal and possibly avoid a longer sentence at a maximum security prison. The tone was optimistic so maybe the mother had a change of heart and was able to convince the husband to drop the charges and Lu was able to avoid prison time. Or maybe Margo got a good lawyer and helped fight to avoid prison time. I feel like since the tone was optimistic that's probably what happened. Maybe she went to jail for like a year or so?

Even if she does go to prison it wouldn't be for long. She'd be charged with 'unlawful imprisonment in the second degree'. Which going by New York law is punishable by up to one year. Given the circumstances (she took care of her, thought it was the right thing to do etc.) she would probably be given leniency. So maybe a few months or just some community service. I doubt the parents would press charges though.