She should get an abortion (unless she wanted to have the baby as her own personal choice)
Carrying a child to term in that situation would be extremely traumatic for the girl, and would interrupt her life in this way. This is a situation that she would be much healthier for dealing with it emotionally first, and then putting it behind her.
Also her mother would suffer if the child was carried to term. Imagine your spouse impregnating someone else! Ugh!
Finally, it could be hell for the baby to be unwanted, conceived through rape. That child has a chance of going to an abusive home. Then the cycle would start all over again!
The biggest favor this girl could do for herself and the potential baby, is to have an abortion. Then the girl would have a chance to carry on a normal life after such a heinious incident.