Joe asks his ex-boss Mary if she is willing to be contacted by future potential employers. What do we call Mary?
"Referee" is one word used for this, but that means "sports umpire" to most people.
"Reference" is another ("Will you be a reference for me"), but "reference" to most people means the action or the text of the recommendation, not the person. Also, "Will you provide a reference for me?" might mean "Will you write a recommendation?", not "Are you willing to (interactively) vouch for me"? (e.g., by phone or email conversation)
What word do we use for this willingness to interactively vouch for someone?