Executive Summary
You have already decided to leave. If you bring up this issue on behalf of this employee, he will have to live with the consequences. He may not appreciate that very much.
Company Culture
All companies have a culture. Some companies are filled with young go-getters who work silly stupid hours and make a game out of who can work themselves the hardest. That may not suit everyone, but it may make for a very productive (and successful) company culture. Some companies are filled with drinkers. Or philanderers. And those may not be right for everyone, but they may make a good company culture for those who fit in.
Companies are a lot like partners in a relationship -- you have a much better chance of finding a better fit than molding the other person in to what you're looking for. I would be very careful about suggesting any change to company culture, especially if it is ingrained and not causing undue problems in the office (especially if it isn't directly causing a problem for you).
Timing
Why now? You have been working there for a significant amount of time, and while you haven't participated, you haven't spoken up or brought it up with HR yet. So why are you so keen on doing it now?
Be very careful that you aren't bringing this up because you feel you have nothing to lose, akin to a Hail Mary Pass. If it is important enough to do, it was important enough to do when there were consequences.
Aftermath
Let's say you do bring it up. Consider the potential outcomes. Here are two in particular that I think merit consideration.
You tell the HR company about this banter. The HR company is torn. On one hand they have been told by an employee that they feel uncomfortable with some of the back-and-forth in the office. On the other hand they may only have your word for it, and you will be leaving to the point where they would be fighting the battle on their own if they did bring it up with the company owner. Essentially, you pass the buck to do something to the HR company.
And another:
You bring up the issue with the management of the company on behalf of this young employee. You say that they are making him feel uncomfortable, and that he shouldn't be forced to participate in that sort of banter in order to be included in the company culture. You have just painted the employee as someone who cannot speak up for himself and who doesn't fit in to the company culture to the people in the position most able to ostracize him entirely.
To return to the timing aspect, if you were confident you could change this behavior, you could have brought it up earlier. Bringing it up will likely cause problems for other people without any choice about it on their behalf.
Suggestion
If the above isn't clear enough, I would be extremely cautious about bringing this up to HR or the management on behalf of another employee. The employee can speak up for himself if he feels uncomfortable, and it is only a problem for HR if an active employee (one with skin in the game) brings it up.
Given the fact that you say you have 'thick skin' and aren't bothered by this, I don't think this is a serious issue to you, but if you think it is then you should address it directly to the owner.
Hey owner, as you know I'm leaving soon. While it's not the reason I'm leaving, you may have noticed that I didn't match the company culture quite as well as some of the other employees. I can only speak for myself, but even joking banter like X, Y, or Z that can be construed as sexist, racist, or homophobic can rub people the wrong way. I have a thick skin and just ignored it, but I just wanted you to know that even if I didn't speak out, I did notice, and as the company grows it may be harder to find people who are okay with that sort of discussion in the office.
The point is that you aren't condemning it, you are only speaking for yourself, and you're explaining why he may want to care about it (if he is growing the office). Don't get in a debate about it, or argue about it (since you say it doesn't bother you), just let him know so that he can do something about it if he chooses.
If your main concern is the coworker, then I would have a discussion with him directly:
Hey coworker, I've seen you chatting with A, B, and C over coffee. Maybe it's just me, but you look a bit uncomfortable when people start talking about X, Y, or Z. I'm uncomfortable with those things too, and I worked here for n years without participating in it. Don't feel forced to mold yourself to these people if you don't feel comfortable. If I can do anything to help, just let me know.
The point is that you are letting him know that others feel the same way, and that he doesn't need to join in. At the same time, you are letting him make the decision on how to proceed, and giving him an opening to ask for help if he wants it.
At any rate, bringing up the issue right before you leave has the potential to hurt your connections with this company if you ever want to come back, and potentially if you want a recommendation from them in their future. Criticizing a company as one of your last acts before going out the door often leaves a lasting impression far stronger than what you produced in the years you worked there. Proceed with caution on that front. Even if you just tell the coworker, he may use that as an opportunity to try to endear himself to your coworkers by throwing you under the bus.
At any rate, how you proceed is up to you. Just consider your motivation, what the potential consequences are, and whether it's the right decision to make in the long term.
