I almost let this happen today, but it's 6pm and I'm ready to enjoy the other half of my life while they're busy wasting their's away in a near empty office.
Edit: To be clear, I don't mind working late every now and then. I could see myself staying one or two days a week tops to catch up on work. Or during busier holiday seasons where it simply doesn't justify having an excess of year-round staff to cover for periods of exception.
Sometimes during the week I am just in the zone and I can't get myself out of it. That's healthy. That's just being productive and taking pleasure in it.
But, staying late under duress or as a result of obvious ineffeciencies that need to be fixed, or staffing deficits is a completely different story. Those need to be addressed in conversation, not in hours where you could be coming home early enough to make yourself dinner, spend time with friends/family, do something fun for yourself.
Also, I actually like my company. The office has a friendly vibe to it, the work I do isn't terribly complicated and makes the day go by quick, and my boss is to my astonishment, actually a really nice guy. I'm genuinly shocked. I have only ever worked for complete assholes.
This small issue with my co-worker is just that. A small issue. But one that's important to take the right course of action early before it becomes a bigger problem.