What do you call a person who is willing to be a reference for someone?

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?

Reference, as in Will you be a reference for me, is quite commonly understood to mean the person providing the reference, in spite of some dictionaries not indicating such meaning. I don't know of any data to confirm or confute your claim that "reference to most people means the action or the text of the recommendation, not the person". I agree there can be confusion, even though "be a reference" implies acting as a reference contact.

In looking at synonyms of reference and referee, I see many words that will serve in specific cases, but no English words that have the specific meaning sought. (In Dutch, referentie means "a reference, person who vouches for another, character witness".) Synonyms of related words include: advocate, angel, backer, benefactor, bonder, champion, defender, follower, friend, guarantor, helper, partisan, patron, protector, referee, second, sponsor, supporter, warrantor, well-wisher. Neological terms include: voucher, referrer, recommender.

(Edit: removed duplicate words from list)

Collins: English Dictionary definition (meaning) of reference:

1. the act of referring,
2. a mention, this book contains several references to the Civil War
3. direction to a passage elsewhere in a book or to another book,
4. a book or passage referred to,
5. a written testimonial regarding one’s character or capabilities,
6. a person referred to for such a testimonial,
7. relation or restriction, esp. to or by membership of a specific group, without reference to sex or age,
8. ◇ with reference to concerning, adj containing information or facts, reference books, ◇ adj referential