testify for (someone or something)
To present evidence or testimony in favor or defense of someone or something as a witness in a trial. The doctor was asked to testify for the plaintiff in the criminal negligence lawsuit against the construction company. I agreed to testify for Daniel, since I was with him the night of the incident.
testify for someone
to present evidence in favor of someone; to testify on someone's behalf. I agreed to testify for her at the trial. Max testified for Lefty, but they were both convicted.