I'm sorry to hear your mom's judgment is clouded right now; she's probably lonely after your dad's death and can't think straight.
You're a good daughter- and I appreciate how worried you are. But all you can do is write her a nice letter telling her how much you love her and how worried you are, and then give her some space to come round on her own.
Hitting is abuse and should not be a part of any relationship. If this man hits her (or you) again, please call the police and don't afraid of him. He's a bully. Then schedule an 'intervention' for your mom with the help of her family and your grandparents to help her see sense.
Always be there for her to lean on. If the situation at home gets you down, you can always live with other family/ friends. But you have to keep in contact with your mom and watch out for her. It's hard to watch someone we love making the wrong choice.
All you can do is advise her, be there for her, and call the police as soon as this man hits her again. And then have her family intervene.
Please also show her the letter you wrote this site and my reply. I hope she comes to her senses in time. Just call the police as soon as he hits her, so he can't harm her any more- as physical abuse starts with only a slap, then it gets out of control.
Also go see a counselor by yourself who will listen and understand and keep it confidential- and give you tips on how to help your mom through this rough patch.