Thats hard. My DH lost his dad in 2000. His parents were married for over 50 years. We have talked about it many time in th epast. HE says he could never imagine his mom with someone else but now that she is in her 80s and alone he feels like she needs someone. He would still have a hard time with whoever she ended up with but as long as he treated his mom well he says he would deal with it.
Its hard, you always see your parents as being together and when something happens when they arn't its tough.
There are far worse things he could be doing than speaking his mind and being rude. It may be wrong to you but does that make him a bad person? If he is good to your mom and makes her happy, give him a chance. You never know you may see why she loves him and wants to marry him.
My family hates my DH so much so they have stopped talking to me all together. I had to choose between them and him. Well he makes me happy. When I am with him I feel like I can do anything and be who I want to be. Sometimes I fell like maybe I did not make the right choose but while I was around my family I had a drug problem and self esteme issues. Since they have been out of my life I have been clean, I lost 80 pounds and feel great. If they only would have given him a chance and seen what he did for me then maye things would be different but they never could. Now they don't have me and were not here to see the good changes in my life.
Give him a chance. You never know.
Sending you vibes and hugs