Actually there is no real binding of B to A. B doesn't depend on A. It is just a "service" available to any object that can see it. B is a "provider" I suppose, but not specifically to A. It may be important to convey the asymmetry of the relationship, actually.

I like that.

You used both "service" and "provider" which I'm taking to confirm my suspicions that there is no single agreed-upon term.