Firing someone for reporting a co-worker’s inappropriate texts and having concerns about whether or not it may be a factor in your opportunities for advancement would be retaliation, and is illegal for an employer to do.

It is a risk, because HR at this company may be shitty, but the law is clear.

HR is there to minimize liability for the company. Investigating questions and concerns is what they do, but not on behalf of the person with concerns - they do it for the company.

Sending sexually explicit texts to a coworker, on its own, shows poor judgement and is unprofessional, and HR can decide if 1) it violates the code of conduct for the company and 2) if there is any legal liability for the company.

And then they decide what to do about it. Depending on what they decide, it may be a request to the asshat that he refrain from sending texts like that to coworkers again, it may be a verbal or written reprimand, or more. Or nothing. At minimum, they’ll likely make sure he’s not part of the hiring committee if OP applies for advancement opportunities in the future.

In the overall calculus, if this is a small company, it may not have a functioning HR department (or, more likely, they have an HR rep whose job is mostly something else, they’re not trained in HR, and only process the paperwork for hiring and separations.) If so, it’s much more risky to talk to “HR”, and it may be time to start looking for work elsewhere - not because OP can’t prove what was happening in the asshat’s mind or because the inappropriate behavior was 2 years ago, but for OP’s own piece of mind and career, and because this asshat may be “poisoning the well” on her advancement.

OP is speculating on what may be happening, has legitimate concerns about it, and has bent over backwards not to jump to conclusions. Asshat co-worker did not take any care in considering how sexually explicit texts to a coworker and subsequent participation in hiring decisions about her would look, or may impact OP. Giving even the appearance of bias or discrimination or retaliation is very not cool behavior that would concern any functioning HR department. It may not rise to the level of firing the asshat, but it is very not cool.