How involved in your child's life is too involved?

When I was in high school, my sister and her best friend had this huge fight and falling out. I guess other girls in the school got involved because several weeks later, this girl withdrew from school and her parents put her in the local catholic high school. I scoffed that I could't believe her parents wouldn't let her fight her own battles and would let her just run away from her problems.

Yesterday, I sat outside talking to a neighbor as I witnessed peripherally two neighbor kids gang up on Bianca and tease her until she went inside. I didn't want to intervene. (I know I'm accustomed to doing that because she's an only child and I feel this urgent need to protect her.) I kept talking and kept an eye on the situation. Bianca came out and rode her bike by herself. A couple minutes later, the girls were sitting on my front lawn and I heard one of them say, "Bianca's the stupidest friend I have."

I can't remember exactly what I said, but it was something to the effect that she wasn't acting like a very nice friend.

As I sat reeling over this situation last night, I was tempted to never let her play with this friend again. I resolved that if she calls, I will say "Until you can prove you're a kind friend, Bianca can't play with you." I want to keep Bianca inside where the world is kind and arrange playdates with kids I know are kind to her.

My question (especially to all you veteran moms out there) is: Am I being overbearing? Should I let her figure this out herself? My guess is if this friend called and asked her to play and I said no, Bianca would be mad at me. Kids are forgiving. I'm not nearly as forgiving as Bianca.

I know I have a lot of learning to do as a mother, but I wish I knew when can I protect her and when should I just back off and let her figure it out for herself.