She's likely trying to attack Luna's reputation. Fan A attack to paint her as a bully failed hard, so hers is likely to paint her a psychopath (what she is as we saw) because after the last one. Luna is being seen as being harassed by Mana's supporters! Remember the Prince (and Mana) were counting on Luna getting upset or panicking. Instead Luna kept her cool and as said, she's being viewed as "minding her own business!" As Mana can only prosper (as she believes) if Luna acts like the "villainess she's suppose to be"
This girl might be acting on her own in case she fails her "precious Mana won't get implicated!" But I will laugh if Off Screen, Luna had noticed the mirror gone and immediately reported it missing with the staff. Meaning her "key evidence" falls apart cause if it was reported missing and Luna has an albi. The question goes "how did she know it was Luna's mirror" along with "funny you just HAPPENED to see Luna do this bad thing and HAPPENED to find her dropped mirror nearby" she could play that she remembered seeing the mirror and MIGHT have confused Luna for someone else. But knowing how Luna handled the last fool, she likely off screen left rope for Fan B to hang herself with (Note this chapter she NOTICED despite trying to act like she was Luna's Entorage, she couldn't control herself and got sighted by Luna displaying her hatred to her view her eyes by Luna, hence my belief she went through her things last night, noticed the mirror gone and suspected so reported it missing immediately just in case, all Fan B did was expose herself as "an enemy within" meaning Luna needs to vet those around her and be concern for the girls since Fan B showed Mana's Groupies aren't above putting THEM in the Fire range of it will hit Luna, and we know how Luna feels about innocents being targeted by self righteous scumbags who convince themselves they are in the right for doing so cause "she's the Villainess" so who cares, he'll they even DESERVE it for daring to befriend EVIL! Never underestimate Zealotry ability to use Insane Troll Logic to justify Evil actions towards the innocent!)