Hello,
1) It is hard to fathom why the college/the employer should retain her original certificate when she has already resigned from her job!
2) The employer has no right to do so even if it meant she resigned without adequate notice. Do issue a legal notice to the employer demanding immediate return of the original certificate.
3) You can also lodge a police complaint seeking intervention to get back her certificate.
Show the police the proof of resignation and the letter from the institute acknowledging receipt of certificate in the original.