it is a very weird situation. Both of you are Christians. If you are a true Christian, you should love her unconditionally like Jesus. However, I have never seen a Christian who love other people unconditionally. It is almost impossible for human being like us.
You already posted your worry on this site. That means there is already a doubt about her in your mind. If you try to ignore the doubt and then marry to her, the worry will become serious and will ruin your life.
What I recommend is to help her to contact her ex-husband and her child. Then, if you can love her and her child unconditionally, then you can decide to propose to her. Or if you cannot love her and her child, just leave her. Please help her first. She is your girl friend and best friend. But we don’t know whether you can share her pain or not even if you are Christian. Before you say you are a Christian, you are a human being and a man who has jealousy and doubts. I have never seen a Christian who could overcome human nature. All human beings are the same even if he or she is very religious. They follow their instinct of self-preservation eventually.