Never.

You read that right. It is never acceptable to reveal another person's sexual orientation or gender identity. You can make a mountain of excuses, but none of them will take away from the fact that you have violated this individual's right to comfort and privacy.

No, "everyone" does not already know that person is queer/non-cisgender.

You may claim that you assumed this person's identity was common knowledge...but you know what they say about being so quick to assume. The so-called closet only exists because there are still intolerant members of society. There are still people out there—religious or close-minded or just plain ignorant—who take every opportunity to express their hatred for and disgust with the LGBTQ+ community.

And yes, your friend might be out and proud in their liberal college town, but they are still closeted when they go home to their conservative friends and family. The point is, know that not everyone in the free world is aware of someone's queerness or their gender—and there might be a reason they don't know.

Outing people can seriously put them in danger.

While they might not be killed or tortured in the town square for their identity, this person could lose financial and emotional connections that are critical to their survival. Parents who don't condone LGBTQ+ identities, for example, may tell their child that they're no longer going to help them with their college tuition payments. They might even completely ostracize them from the family and forbid them from ever going to any future gatherings. Aside from that, this person might have their fellow churchgoers turn against them and prevent them from enjoying their religion the same way. Thanks to your careless reveal of this person's identity, influential people that once brightened this individual's life will now be motivated to destroy it.

Coming out is relatable to a lot of people, but it still is personal.

"Coming out shouldn't be a big deal anymore." And you're right about that, but you're wrong if you think the best way to promote positive change is to bust down the closet doors yourself. Whatever the reason, this person has chosen to stay hidden for now, and you need to respect their wishes as an individual and keep their "secret" to yourself. Because it is their identity and their life at stake in the aftermath of exposing it, this person's decision to come out or not come out is entirely their own.

You could ruin this person's relationship. 

If your closeted friend is secretly dating someone of the same sex, don't take it upon yourself to shed light on their private lives. Outing them will not only expose their individual identities, but it will also publicize their romantic and sexual relationship. Their partner might not mind, but they also could be so distraught over being outed that they break up with their S.O. to dismiss the allegations altogether. Then you've gone and ruined that person's relationship with themselves, their world, AND their lover all in one shot. Way to go, home-wrecker!