I don't know enough about the context of your relationship (whether friends in workplace, school, etc.) to know your next step (as in if there's a boss you should report to, a school administrator, etc.)

However, I do know you probably did see what you think you saw. The way you describe how he evaded rings true, too, Neo. I'd say you're not seeing things. My two cents.

Yikes, that is a problem and I agree you are likely seeing what you think you are seeing. I liked this article on helping someone who cuts. I think it makes some good points, including making sure you are ready to help that person and you are in a good place.

http://www.wikihow.com/Help-Someone-Who-You-Think-Is-Cutting-Themselves

When I was at my worst I cut, but mine was more out of self-hate for myself and punishment for not being able to cope with the stress and situation I was in. Plus it was always when I had been drinking and no longer cared about anything. Pretty sad now that I think about it, but the pain was a huge distraction from the rest of life's fun and games.

I think honesty with the person might be the way to go here. Please know there is likely to be a lot of denial when talking to the person as it is a highly embarrassing thing, which no one wants to admit too