Can an employer rescind an offer letter based on a negative recommendation from someone you did not authorize to provide a recommendation?

1) IF the offer was noncontingent and you indicated that you had accepted it, then it may not be rescinded. If you had not accepted it, the company might be able to argue that you had in fact rejected it.

2) There is no law saying that you can only get "references" from those that  you authorize--anyone may be asked for their opinion of anyone.

3) If the person made untrue statements of fact about you, then you were slandered and you could sue them. However, personal opinion is not  actionable. For example, if the person said, "John Doe stole from the company," "John Doe lacks the qualitifications to do his job," "John Doe harassed other employees," "John Doe was chronically late" or other factual statements and they are false, you could sue. If she said "I hated working with John Doe," or "I'd never hire John Doe again," that's not actionable--that's just a personal opinion.