I wonder if she would respond to some (gentle) teasing about L? How L seems to tell DD what to think/do - hasn't she got her own brain? That you heard L is a mean girl and she should think about people calling her a mean girl too... Push it a little bit further so DD can see the longer term effects on her actions.

I feel a bit for L, but I also think she is going to dig her fingers right into your DD. I bet she doesn't have many other friends acting like this or having a mum around a bit more to guide her behaviour. Hopefully once DD sees how other people view this behaviour she will back off of her own accord.