NPC has collegiate Panhellenic advisors, and campuses can request that they come in to their campus to look things over. This seems like the kind of situation they were designed for.
I'm not sure how to contact them - perhaps your chapter's advisor does, or the Greek advisor, or the local alumnae Panhellenic, or even your sorority's nationals. If all else fails, try NPC nationals.
Honestly, I don't think dealing with this locally is the best idea. As PsychTau pointed out, several NPC compacts are being violated with your campus' current procedures aside from just this overbidding - and sorry, your campus' Greek advisor SHOULD know that, so he/she does bear some fault. No joining elsewhere the first semester after depledging is just about the most basic NPC rush rule there is.
Also - the chapter in violation should inform their nationals (I know, they don't have much incentive to) - but their national HQ may very well have a standard response to a situation like this; they may have dealt with it before.
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