I'm in agreement with @Jake - but wanted to give more detail and some options.
First - Management is responsible for unreported HR violations
Would you think well of a police officer who watched a mugging happen but who didn't intervene because he wasn't the guy being mugged?
To some extent management owes you more than just trying to help workplace be more tolerable for you in ways that publicly visible to you. If a hostile work environment is forming (sounds like that's the case) then management has an obligation as a legal representative of the company to get help, report incidents and engage legal, HR or more extensive management when they can't do it alone.
It sounds like your manager has been unable to directly prevent the harassment of females, covering it up wasn't his best choice.
You also are only hearing some of the story - what you have seen (your manager being publicly supportive), what your manager said (that he can't talk), and what his admin claims. This is a lot of nebulous information. You really don't know the stories that HR is getting right now, from either your harassers or from other people in the company. Don't assume you know the whole story.
What you CAN do...
Write it down - write down all the cases you can think of where you have been harassed or watched another employee be harassed. In particular, try to jot down - the date, the people present, the location, the behavior (your interpretation), and any objections raised by yourself or others. As you say, the chances are good that many of these recollections will include the good manager stepping forward and saying the harassment was not OK.
- Get a friend to edit it, often writing like this is long and ranty - make sure that it's tightly honed for a corporate environment.
Communicate, in writing, to HR saying that you want to be clear on how supportive this guy has been in public. Provide an attachment with your writing.
Get other people who feel the same way to also do this.
The guy may have used poor judgement in not escalating, but it sounds like he's tried to set a tone where others should know that this isn't condoned behavior. If he's getting into trouble for allowing a hostile work environment, this should at least counteract the assertion that he's been implicitly condoning the harassment.
If you don't get clear satisfaction from HR, there is usually an anonymous hotline you can use.
Keep in Mind
When issues like these go hot and legal, they are often EXCEEDINGLY private. The HR reps dealing the problem are also legally liable if they damage someone's public reputation, and they will be very very cautious in sharing details with you. The best you may get is "thank you for your input" and if the guy is fired, it may not ever be clear why.
Which means the person could be fired for something totally untrue (unlikely, if he can prove the case in his favor), or the reason he's getting fired could be completely unrelated and completely justified. Or it could be that his family life just exploded and you'll never know.