This is a tricky situation. My Advice is based on how far you and your mother are willing to go to get justice.
One problem is that you are far away from the situation and will have to remote control this.
Your Mother has to be sure that the child she is pregnant with belongs to your friend. This she will know if she has not had any other sexual intimacy with another man. She also has to decide how deeply she loves your Friend and is willing to have his baby.
The Law is on the side of your Mother and any complaint from her to the Police will put your friend in jail. He will be forced to marry your mother. But is this what she wants? If your friend does not love your mother, then all her life she will be unhappy.
Talk to your friend and see if Law threat works. You may have to involve some elder or respectable Member of your Society/Family in this matter. If your Friend is just not willing to accept your Mother as his wife then it would be better to abort the baby. Check if it is still medically possible as once the time has passed the Law will not allow it.
If your Mother wants to keep the baby, she will need to seek financial assistance from your Friend. Make a legal agreement. Just by word of mouth is of no use in the future.
Do not panic. You need to be a support to your Mom at this critical stage in her Life.