We all know that a man has the right to marry 4 women.
My question comes with an example. Let's say a man gets married and has income only for one wife. After some years he meets a younger woman and behind his wife's back he makes a baby with her without being married. When he sees that things are hard on him then, he tries to cheat the wife with lies so she will leave him. What is the position of his girlfriend with the child? Is she liked by Allah and especially when she is a Christian and does not feel she did wrong stealing someone else's husband?

What he did is haram by all means, and the child is illegal child. If the girlfriend was part of the cheating action, then she shares the sin with him.