If a 15 year old girl gets pregnant by a 19 year old guy what can happen if the mother consents?

It depends on where you live. Even in the US, the laws vary by state, but I'm not aware of any state where it's legal for a 19-year-old to have sex with a 15-year-old child. Even if she screamed 'yes' at the top of her lungs, it's irrelevant, because she CANNOT legally consent to sex. So, the 19-year-old has broken the law and can face some serious consequences for it. The DA can charge him even without the consent or cooperation of the girl and/or her parents. He needs to be talking to an attorney.