what would you do

That's a lot of questions, and they really depend on the person and exactly what they're doing and why. Compassion is generally the better method, but depending on circumstances "tough love" and showing them that what they're doing is upsetting to others can also be a potentially better way to go. They might think it's the "only way," but that's not actually true. Someone who cares about them should help them to learn how to handle their problems without doing harm to anyone. Whether that's looking up the methods of distracting from actual self-harm (like ice, and rubber bands), or helping them to find a counselor of some sort and encouraging them about seeking that help and doing positive things. Many of us have smiles that are often only masks. So what? That's not important. It sounds like you're twisting this to be about you rather than about them. That's not cool. It's not about you. Maybe part of the reason they're not telling you is exactly what you are demonstrating here, that you'd be too upset and conflicted about it all and thinking about what they've been doing to you rather than you being concerned about them. Sorry if that bothers you, and it could be wrong, but based on this post alone, I'm just calling it like I see it.

I see... I don't think you can really get in someone's head like that though. Not unless you have been in that exact situation, and even then, we all are unique and think our own thoughts and react in our own ways.