Avraham Yom Tov who was close to Rav Ben Tov for many years, related: I knew a person who was an underworld crime figure, Hashem Yishmor. A inhumane and aggressive person who used to beat his wife. They didn't have kids.
I once told him "Come to my Rav. What do you care? What do you have to lose?" To my great surprise he agreed and so we went.
The Rav looked at him with a loving expression and asked him "Do you keep Shabbat?" I interrupted and said "What Shabbat?? He is faaar, faaarrr." The Rav turned to me and asked me not to intervene. The Rav asked him "Do you eat kosher?" Again I couldn't contain myself and said "There is no Shabbat, no Kashrut, no boundries". The Rav said "It will be OK".
"Do you want children?" The Rav asked. "Do what I say: Take your wife to a woman who is an expert at hilchot mikva, she will tell her what to do, you also do as you are told and follow the rules and in two months your wife will be pregnant and she will give birth to twins."
After everything the Rav said came true the couple started keeping more and more mitzvos until they did complete tshuva. Today he sits in kollel and learns full time.
צדיק המזוזות עמ' 127-128