For the most part having your mother be your best friend as an adult is healthy. But I think it also depends on other factors. Now, I did read once how your first few years of marriage to move far away from your parents to make a life with your husband and not tell your parents everything little thing you fight over.
I'm very close to my mother too. I live 1.5 miles away, I'm 37 years old. My mom is my best friend too. I was really close with my mom growing up, I'm one of 4 girls, #2. She is a middle child. I always felt great going to her. I'd say in my late teens to early 20's I didn't, I think I knew I was making bad choices and new she wouldn't approve. I look back at the mistakes I made, when I thought I knew better than everyone else what to do with my life. Maybe those are also building stones for my independence. I know it could have saved me a lot of stress if I would have just listened.
I know my mom means well, but she has also overstepped her boundaries and I've allowed it. My daughter who is almost 16 is very close with my parents. I want her to have a very open relationship with me. We did have a hard patch but in the last 2 years it's been getting better. My daughter does open up to me for the most part. But she also knows I'm her mother and will be that before being her friend. I have boundaries. My mom thinks I'm too hard on her a lot, and not understanding, and she'll throw how she wasn't that way with me. My daughter takes after her dad a lot and I do think my daughter needs hard rules because she will try to move the boundary lines. Don't get me wrong, she's a very smart girl and nice and caring to a lot of people. I guess my husband has also shown me to be a parent first then friend later in life.
I think it's pretty healthy to have your mother as your friend. Just don't tell her all your husband's short comings or every fight you have. You shouldn't tell your girlfriends this either. I want to have a great healthy relationship with my daughter and sons when they are adults. I hope they would consider me their friend too.