That depends on the circumstances. It's not at all unusual (though becoming rarer) for a brown-, purple-, or even blue-belt in BJJ to teach. It started out that way because there simply weren't enough black-belt folks available in many areas. It continues mostly because it worked before.In today's world I think you need at least a black belt to teach. In my first MA class over 40 years ago, I was taught by a yellow belt with 5 years of experience, while my teacher watched. I think it was as much about him learning to teach as it was me learning. I have 12 years of experience in MA and have coached many sports for many years.
Today I am wearing a white belt, I tired to help a white belt, the response was the black belt did not tell me that. In her mine how could a white belt know any thing. I just smiled and kicked the bag slowly and hoped she was paying attention. The young black belt method of teaching was kick the heavy bag and said do it like this and make sure to pull your toe back and walk off to watch the higher belts.
I real like how Kung Fu long fist and Alkido had only black belts and white belts. White belts judged each other by their ability, not by a colored belt. Last night our teachers were out of town, I was not wearing a belt and we had a teacher I had not seen before, half way throw the class she asked if I had been a black belt somewhere else, she was surprised when I said no. She was a good teacher and showed me respect. She noted I did the form slightly different, no one else had noticed. My blocks flow across my body hands open until the end of the movement. The hands move faster if not in tension. Holding a hand with tight fist slows you down.
I know how to teach better than most teachers, because of my many years of coaching sports. But without a Black Belt no one would let me teach and I would not ask. I think people need the safety of an organization to say the teacher knows what he is teaching. Where as I can tell with in the first 5 minutes without looking at a belt to know if he is a teacher.
And it also depends on how people are educated about what the rank means. Where BB is sanctified, you're more likely to get that kind of reaction where if it didn't come from a BB, it's probably not right.