My sister is getting bullied by her 'friends'?

My sister is 11 and she has this friend is SO mean to her! She always says my sister is always acting 'innocent' and just lying so people would believe her and she actually convinced many people of that! Yesterday, I saw my sister and her 'friend' arguing on Facebook and the 'friend' even said she could turn all my sister's friends against her as they hang out with the same group. I really want to beat those stupid girls up but that might make it worse for my sister. What should I do? Please help me!