Where is she? or Where is she at?

Both are acceptable. The first is uncontroversial and I do not discuss it here.

It is common usage to say things such as "She is at the cinema", "He is at the bar", "She was at the races", "He is at home". The "Where is he/she at?" questioning construction parallels these statements so finds itself in frequent and acceptable usage.

The "at" is redundant. It is not needed because the questions could be more concisely put as "Where is she/he?".

This redundancy, and the efforts of seventeenth and eighteenth century grammarians to align English with Latin, lead some people to say it is ungrammatical to end with "at". But it is not: see https://www.merriam-webster.com/words-at-play/prepositions-ending-a-sentence-with for a reasoned historical discussion.