My friend was terminated for harassment of an employee who conducts the same behavior, is this legal

My friend (Joe) was just hired at a new job on August 8th. Over the course of four days of training he was subjected to listen to a particular employee (Jane) curse and make offensive comments regarding the people she had spoken to on the phone (calling one particular caller a "F**king dumb*ss").

Then, on day 3, it was just Joe, Jane, a third associate (Sheryl), and David’s cousin (Donna) in the room together and as Sheryl left the room she said "Oh, I just saw Dick" (referring to another employee). After the door was closed and only the three employees were in the room, Joe made the comment "Oh, did someone forget to zip their pants?" Over the course of the next several minutes the three employees all took part in the joke, Jane being one to say "I don't like paging him because I feel like I am yelling for dick (meaning sexually and while she was laughing).

During all of this, no comment was ever made to Joe that what he said was inappropriate or taken offensively. However, the next morning, he was pulled into his manager’s office and told that "someone" claimed to be victim of a hostile environment as a result of the joke and that "per their policy" he was to be terminated on the spot.

We have already spoken to his cousin to ensure that she had not taken any offense to the joke and were assured that she was not the "offended" party, leaving only Jane.

My question is this; can the "offended party" claim a hostile work environment to get David fired if she herself encourages such an environment? It is well known that Jane speaks “like a sailor” on a daily basis, and I have even been in the room during many of her hostile comments.

Furthermore, after hearing what happened, I immediately spoke to management and HR regarding my feelings of a hostile work environment as a result of Jane’s language in the office and the only response I was given was that HR will conduct an investigation but should the investigation result in disciplinary actions that I would not be privileged to know of them (insinuating that an alternative outcome may occur even though the situations are identical). Furthermore, this would allow them to avoid conducting the investigation and not penalize Jane for her behavior.

Lastly, just as a note, Joe was a brand new employee, I have only been there 3 months, Donna has been there 10 years, and Jane has been there even longer than that. Regardless of the employment duration, is it not required that management react to each occurrence of a hostile work environment (my complaint versus Janes)?

There are tons more details but I just wanted to get an initial opinion and can supply more detailed descriptions at a later point.

Thank you for any advice you may have.