Is it obligatory to inform the maternal uncle that his niece has been expelled from the institute because she was associating with a deviant young man who was exploiting her?

First: Intensify your supplications for her guidance and adherence to righteousness.

Second: Advise her with kindness, from a standpoint of compassion and mercy, while observing the Sharia-compliant restrictions, given that she is not a mahram to you.

Third: If she does not benefit from the advice, subtly threaten her with informing her father. If the hint is not effective, then state it explicitly.

Fourth: You must guard your tongue and accustom it to good speech, and not describe the girl as foolish.