That's not blackmail. That's called being an *******.
What he said. It's not blackmail, and he's in no position to make demands. He only does it because he knows you won't retaliate in kind.
Let me tell you a story about someone similar I once knew. I think this guy is considerably worse than your friend, but they do share common traits.
This guy, we'll call him Ass, is what I like to call a sympathy magnet. He was an employee of mine, referred to me by someone I explicitly trust (though tends to be a little too trusting of other people himself). Ass was one of those people who always has a sob story, always in trouble, always asking for help, and always, ALWAYS has an excuse on hand.
Now Ass wanted to buy a nice bed for his wife. Since he has terrible credit (which should've been the first clue as to what I was getting into, c'est la vie), he needed me and my solid credit to cosign for a $1500 loan. At first I refused. No. I'm not gonna do it. I didn't want to do it. But he kept hounding me, over and over and over again, until, finally, I did it just to shut him up. Got him to sign a little waiver, had it notarized, all that good stuff. Then off to the bank we went.
Long story short, Ass is late with the first three payments, then, as far as the bank was concerned, dropped off the face of the earth. For two months, they couldn't get ahold of him. Of course the remaining debt falls on my shoulders.
Know what I did? I bothered him about it at first. Got the usual "Oh, I'll pay it. Just do it this once. Blah blah blah. I'm hurtin', man" both times. I remind him that I'm writing his checks, and know exactly how much he's making, and that he could pay it off quickly. That of course leads to more excuses, "other bills" and whatnot. Needless to say, I got sick of them pretty quick, and decided to do something about it myself. I call the bank, and tell them flat out I wouldn't pay a dime until they repoed the bed and auction it off. Once they did that, I'd pay the remainder.
I could accept having to pay some. It was my mistake to make, and that was the penalty for my stupidity. But I sure as hell wasn't going to let Ass sleep in his nice new bed that I paid for. By that point (and for other reasons), I could barely even tolerate the guy.
Sure enough, not even two weeks pass and he pays the entire damn thing off in one lump sum. ONE. LUMP. SUM. He could afford it. He just didn't want to pay for it.
...then he tried getting revenge for putting him in his place, but that's another very long story for another time.
The point is, as long as you don't do something about it, as long as you continue to let it slide, he'll get his computer, and you'll get nothing but heartache and pain. You need to take some action, or shed your tears, and move on.