We are going through this, so I'm glad you've posted. I think intervention is necessary when it seems like your child doesn't have the skills to handle a situation.
In my 6yo DD's class there are three year groups in one (weird hippy school) and her "best friend" is 18 months older than her and a lot more savvy/mature.
DD is pretty confident and won't always do what she is told, at which point the BFF is "mean" to her. When DD drifts away to other friends, the BFF is nice to her again. I have done what others have suggested and discouraged the whole BFF thing, and talked about how friends are nice to you ALL the time, not some of the time.
I avoided talking to the teacher until this week when said BFF encouraged another classmate to put his hand down my DD's pants. The school has taken it very seriously, thankfully.
However instead of this being a catalyst to branching out to new friends, DD is saying that her "friend" has apologised and promised never to do anything like this again. So our talks continue, and I'm keeping a close eye, along with the teacher.
Stressful, and uncharted territory for me.