I think it depends on what your definition of crazy is, and how long it's been since high school.

If the girl is schizophrenic, or has manic depression or something like that.... first of all, she may be medicated and have her condition under control. If that is the case, the only thing you're going to do is embarass her and make her miserable.

If you just mean that she's super slutty or something... well... that's what new member periods are for. she'll show her true colours in time, and whatever form of Standards Board this group has will deal with her appropriately.

HOWEVER. If she's participating in illegal or dangerous behaviour (doing or dealing drugs, being physically violent, committing crimes, etc) then yes, I do think you have a responsibility to tell someone. However, I would go to the chapter's advisor, who can look into things and find out her deal without getting the chapter involved until it's necessary.

Otherwise, I say, stay out of it. People change between high school and college, and especially in the first year(s) of college. This girl may be totally different than how you knew her.... I would give her the benefit of the doubt that she has come around.