A female professor was found responsible for sexual harassment – what can happen to the signatories of a letter in support of her?
Could such professors who signed the letter be at risk of losing their tenure, because the letter could now constitute a form of retaliation and defamation of the victim (the male student)?
Any answer to this question would be pure speculation at this stage.
Like most US universities, NYU's tenure policies are publicly available. Their Statement in Regard to Academic Freedom and Tenure describes what is considered cause to revoke tenure, but as is common at most institutions, the language is very general:
VI 2 a) [Adequate cause] Adequate cause includes (but is not limited to) one or more of the following: incompetent or inefficient service; neglect of duty; repeated and willful disregard of the rules of academic freedom as set forth in this statement; physical or mental incapacity; or any other conduct of a character seriously prejudicial to his or her teaching or research or to the welfare of the University. [Cf. University Bylaws, Section 92, Removal of Tenured Faculty and Tenured Librarians.]
There is no specific mention of "retaliation" or "defamation" as causes. So, the question would be whether signing this letter constitutes "conduct of a character seriously prejudicial to his or her teaching or research or to the welfare of the University" (or one of the other things listed). That decision would be up to the administration, the Faculty Tenure Committee, and very likely the Tenure Appeal Committee. I don't think we have any way of guessing what they will do. However, the sheer number of layers of procedure needed to revoke tenure would at least suggest that a lot of people would have to be awfully motivated to make it happen.
Could such professors who signed the letter be at risk of losing their tenure, because the letter could now constitute a form of retaliation and defamation of the victim (the male student)?
To assist in answering this question I have edited your question to add the text of the letter. Most of the letter is not defamatory, and consists primarily of a character reference for Prof Ronell. However, there are a few parts in the first paragraph (the highlighted parts) which claim that the complaint is malicious, and this could definitely constitute a defamatory statement against the male complainant in the case. It would be open to the complainant to sue the professors who wrote the letter, and they would have standard defences available to them. This includes a "defence of truth" which means that they would be given an opportunity to prove their assertion that the claim was in fact malicious (and if they could prove this on the balance of probabilities it would constitute a successful defence). Given that the university has found that Prof Ronell did commit the conduct that was the subject of the complaint, it would be hard to prove that the complaint was malicious.
Note that in a defamation claim there would also be an issue about publication of the letter. The letter evidently was not intended as a public document, and was only sent to potential signatories and the two NYU staff to whom it was addressed. It was made public by people other than the signatories, due to its newsworthy character. If the signatories were sued for defamation they would probably try to defend the claim on the basis that they had not intended any publication of the letter. (Some of the signatories, such as Jizek, have made subsequent public remarks on the case.) In defamation actions you can still get in trouble for non-published speech that is sent only to a smaller group of people, so there would probably be some argument on this.
As to retaliation, that is a bit trickier. The NYU Policy on discrimination and harassment can be found here, and it states that it applies to all employees of the university. In regard to retaliation against a complainant, the policy states the following (emphasis added):
VI. NON-RETALIATION
The University will not in any way retaliate against an individual who reports a perceived violation of this Policy, participates in any investigation, or otherwise opposes perceived discrimination, harassment (including sexual/gender harassment), or retaliation, including as a witness. It will also not retaliate against anyone associated with the individual who engages in such protected conduct, such as a family member.
NYU further will not tolerate retaliation by any employee. Retaliation against anyone who complains of, testifies in, or assists in an investigation or proceeding involving discrimination, harassment (including sexual/gender harassment), sexual assault, or retaliation is a serious violation of this Policy, as well as federal, state, and local law. Anyone who believes they have been subjected to retaliation should report the matter immediately according to the same procedure provided in this Policy for making complaints of discrimination, harassment (including sexual/gender harassment), or sexual assault. Any person found to have retaliated against another individual will be subject to the same disciplinary action provided under this Policy for other violations.
Based on this policy, it would seem to me that there is a strong prima facie case for retaliation proceedings against any of the NYU professors who signed the letter. It explicitly accuses the complainant of maliciousness, which would imply that the complaint is false.
Note that the complainant, Nimrod Reitmann, has filed a lawsuit against Ronell and NYU in relation to this matter (see here). The article reporting the lawsuit does not mention any action for retaliation relating to the letter of support, but it would seem to me that this would be a possible cause-of-action that could be added.
It has potential for defamation, because the complainant(s) or their activities were twice labelled "malicious." The actual merits of the case would depend on too many facts that are not present in the post.
The letter by itself probably does not have potential for retaliation, because the authors do not seem to be in a position to effectuate any retaliatory conduct.
Anyone interested in pursuing or avoiding a legal claim should consult a lawyer, though. No post or comment, regardless of how knowledgeable, can convey sufficient information to be relied upon for something that consequential.