Is it a sin to divorce someone because they were violent?

Question:

Today I was researching on why people divorce each other and found your web site giving sound counsel to many. I do also have a question regarding marriage violence.

I have a friend who had to flee home because her husband was violent, he had beaten her in front of her daughters. He had invited women to their house to stay while she was gone visiting her mother. My friend had communicated with him that it is not good to have lady friends over while she's not living in the house, but he did not listen and have continued to do that although he did say he had never had any relationship with them.

Later in the marriage prior to her fleeing home with her daughters was because he turned violent and started teaching his daughters to hate other countries and only his native country was the best. He even taught his older daughter to mistreat the younger daughter because he didn't like the younger daughter to cry a lot and she cannot talk clear. The wife had tried to communicate and hopefully, he would change, but watching daily her girls being taught wrongly and he grew angrier and constantly wanting to hit her. She finally decided to move out with her girls, and let him visit them, but when he visited his daughters, he ended up taking them away, did not allow them to go to school and daily took them to his workplace. For one month he never bathed the girls. When my friend finally was able to find her girls, she took them and fled.

After leaving him, he sent her many emails, taunting her and telling lies to many people, but she never said a thing to complain about him, she just prayed and trusted God, but finally, she made a decision to divorce him. She never spoke ill of her husband to her girls. I had her and her girls live in my house and I witnessed her daughters being so messed up in their values and how they treated each other. I knew my friend whom we've worked together would not do such to her own family life nor her girls. I watched how she raised her girls in my house I knew it was the only choice she could make after years of trying.

My question is this, is this divorce consider a sin? In such case, I know for sure my friend wasn't committing adultery, and I can't be sure he was either, I thought maybe he just didn't set his boundary correctly but unwilling to consider the sanctity of marriage and telling lies and twisting his own children's value under communist false propaganda plus beating her while she's pleading to communicate privately apart from causing fear to her girls. Well, their divorce has just become official two weeks ago, and even at his latest plea to the attorney and the judge his paper was still filled with misleading information and lies about her. Would this consider a sin to divorce? If adultery was not the case in this divorce, can they remarry someone else should they meet someone new in the future?