Ford's lawyer confirms police served his client with a subpoena last week.
The mayor's friend Alexander (Sandro) Lisi faces charges of extortion related to a video that appears to show Ford smoking crack cocaine.
Word of the video sparked a firestorm of scandal for the mayor, who went into rehab earlier this year.
Ford will testify at Lisi's preliminary hearing.
His lawyer says it's not clear what information the mayor has that could bolster the case against Lisi.