What do you do when parents fill your ears with horrible things about co-workers- stories about how they don't grade assignments, or how they sit behind their desk and rarely interact with the kids. You believe that some of it is true based off of the kids (and their skills) that you inherit each year from his/her room. Do you tell your Asst. Principal about it? Do you gloss over it with the parents?
I agree you should go to the individual. People talking gossip probably have something bad to say about everyone, including you, and not just the one person you heard about.
I agree I would talk to my co-worker in private about what the parent had to say. My co-worker may have valuable insight on that parent's child learning skills.
I would take the parent's concerns with a grain of salt. I'm sure that there is some legitimacy behind the concerns, but we know that oftentimes parents see and hear what they want to. I would evaluate the students on my own, see where they are at compared to their peers from other classrooms. If it turns out that the children are lacking significant skills, I would go speak to the teacher about how they presented the information, what lessons were used to practice that skill, and maybe bring up using more rigor. If it continues, maybe speak to administration.
I wouldn't put that much effort into parent's gossip. Simply state what and how you do things in your classroom. I can't imagine observing by co-worker class uninvited to tell the admin. her teaching methods are incorrect.
I think as a fellow teacher, it would be unethical not to do anything...which is what some teachers might end up doing.