If someone else has a mental illness certificate and a copy of their ID card, they cannot sign on behalf of others. According to general legal provisions, possessing a copy of someone else's mental illness certificate and ID card does not constitute signing on behalf of that person. Signing is an expression of personal wishes and behavior and needs to be done in person. In law, a signature is considered a legal act that represents a person's acknowledgment and commitment to the contents of a document or contract. In order to ensure the validity and legality of the signature, it is usually required that the signature be completed in person. If someone wants you to sign a document on their behalf, it's best to ensure their involvement and authorization.