Every once in a while you'll run into someone like that, for whom work is their life.
I've always had passions and hobbies outside of work, and long ago came to understand that I'll never share that attitude. I don't feel guilty about it, but I also understand why that mentality is attractive to management.
I learned that the hard way at my first job. Our senior dev left, and my manager chose to promote my team-mate to the position despite the fact that he had a lot less education, or experience than me. In fact, I was sort of mentoring the guy, yet he got the raise and the promotion. The reason why is because he had moved there from out of town to get the job, and management knew that his financial concerns would make him think twice before switching jobs (especially after offering him money he couldn't possibly get somewhere else with his experience level).
I was upset for a bit, then realized that it was simply good sense for them to "buy his loyalty" in that manner. I had a lot more employment options, and they knew I would likely not stick around. Why promote someone who's likely to leave only a few years later?
That's the way the world works, and you shouldn't take it personally. It'll also be very difficult to communicate to management that this is "unhealthy". They're probably aware! But they're also reaping in the benefits of this person's obsession. Why do you think they're promising him a raise? They're buying his loyalty.
They know that you won't put in weekends, even if they were to give you a raise (you have family, friends, and hobbies you wish to engage with). But this guy is destroying his health for the mere promise of a promotion/raise. You can't compete with that.
You should still, however, bring it up to your boss. I would spin the issue as one of oversight. Explain that this new teammate is making decisions, and implementing code without team oversight, and that this will have unexpected, and likely unpleasant consequences down the line.
If the issue is raised of why the rest of the team does not exhibit similar levels of "commitment", I would clearly communicate that one of the reasons you like your position so much is the excellent life-work balance it offers you, and leave it at that. If you're asked to work late a particular evening, don't be afraid to push back:
Would those hours be paid as overtime, or offered as extra vacation to be taken at a different time, boss?
You may want to speak to your team-mates about presenting a unified front to management. If one person is going to simply fold and start working unpaid overtime, then others will have a much less tenable position when it comes to pushing back against those requests.
If you start noticing that the company is taking a definite turn for the worst as far as the expectation of working for free is concerned, you may wish to start looking for a new job, as management is clearly too greedy and self-serving to give a damn about you.
