A witness is not required for the signing of a power of attorney in Texas. But, the signer must be present with the notary to acknowledge that he signed it. If the signer is in prison, the power of attorney will have to be signed and notarized at the prison.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
The power of attorney form can be mailed to the prisoner at the prison and he can sign there in the presence of a notary. Most prisons have notaries.
This answer is for informational purposes only and may not be relied upon as legal advice.