if some person you dont know on the internet says they are going to get an abortion do we have an obligation to say something?

In general, I'd say no, but there would certainly be some kinds of circumstances where I might say something.

How well do you know the person?

The fact that she is posting on the internet is a cry for help.
Approach the person with understanding and compassion. Speak love. Jesus loves the person as well as the person she carries. Express the fact that, no matter the circumstances, a child is a gift from God himself. This child is an eternal blessing straight from the throne of God! Every life has a purpose ordained by God. God will make a way. Mother Teresa said, 'There is always a way'. She has been given the gift of life. Now she has the opportunity to give that gift to her child. Speak love, not condemnation. Focus on the fact that the child is a person that deserves a fighting chance. Provide alternatives. Suggest a Christian ultrasound clinic and counciling. Very often when a woman sees the child through ultrasound, they are overwhelmed with the fact that a new life is growing in them. Suggest adoption through a reputable Catholic/ Christian adoption agency. My brother was hard as nails and never listened to anyone at that time in his life. I saved my niece's life, his now beloved daughter, from abortion by offering to raise her. There are lots of married couples that would gladly raise the child.

My prayer is that this one life will be saved to glorify God in the land of the living.

i dont know her at all. it was a post i saw in a depression group on facebook.

it was a few days ago i saw the post. i feel bad.

You are not responsible for another person's sin, especially if that person is a complete stranger.

There is an irony about a depression group that makes you feel bad.

Perhaps tell her that if she would like to PM you, you can talk. As you know, most women are pressured into abortion by the father, by parents, or the feel forced due to lack of resources. You may be able to help her see that there are resources, loving couples with whom she can place her baby.

A good secular resource is http://www.feministsforlife.org/you-hav ... r-choices/

The other reason is pressure from the medical community due to a serious prenatal diagnosis. While that is more difficult to talk about, you can direct her to http://www.benotafraid.net/