Tinks i have been through similar to this with my DD around the same age, and im with Junglemum dont say anything to her about (L), i use to say things like do you think what you did is very nice? how would you have felt if it was done to you? Im sure when you have a big think about what you have done you will be the better person by saying no to (L) next time.
I also used the what do you think Ma (my mum) would think if i told her about your behaviour? I dont think she would be very proud of you now. My DD is very close to my mum so i got mum to talk to her many times about different things.
I also think discussing things without actually lecturing her is the way to go. I would bring things up with DD randomly whilst driving etc so it was a more casual convo more than sitting her down one on one at the dinner table IYKWIM