Yes, "Conspiracy" is stretching the letter of the law, but I'm glad they found some statute to charge him because he is clearly guilty. (With the usual caveats, i.e. assuming the facts are as presented.)
To get the BDSM elements out of the way, because they are legally irrelevant, suppose someone was contacted online saying "I want to set up a stunt, I want a complete stranger to come up to me and my mates in the street and shoot me with a dummy gun, you can pick up the gun here and find me here," and the gun wasn't a dummy and he shot the victim dead. While he might be let off Murder One, most people would agree that (a) he deserved a jail term for being irresponsible enough to lend himself to such a stunt without checking more carefully, and (b) the person who set it up was at least equally if not more guilty.
I have, on occasion, been contacted by people who sounded too good to be true, and I've usually gone along for the ride while bearing in mind that they probably were. (The one I really hoped would turn out real claimed to be a lesbian masochist who wanted to be tortured and raped by a man because it was the most horrible thing that could happen; it was a lovely idea, but when she didn't show I wasn't surprised.) If one of them had suggested I start by breaking in on them without any preliminary contact, I would have enjoyed discussing it as long as they wanted to keep the fantasy going: but no amount of lust and fantasies would make me crazy enough to do it.The rapist, on the other hand, while definitely guilty of rape, should also be charged with criminal stupidity. How stupid does one have to be to realize that the person you think you are chatting with, especially when talking about a potentially criminal act, may not actually be that person? We see many reports of similar actions taking place. I'm sure this wasn't the first time for something like this. While he was tricked into the act, I have no sympathy for him, and if placed on the jury would have no problem finding him guilty.
I recall an even nastier case some years ago in the UK where a husband with some kind of sick urge for revenge brought a bunch of yobs back from the pub claiming that his wife wanted to act out a gang-rape fantasy, and don't worry if she was tied down and screamed, it was what she wanted. The rape convictions were an open and shut case, but their sentences were reduced (I forget how much) because the court accepted they were honestly misled by the husband, who bore the bulk of the guilt. But a lot of people argued at the time that they should have been more cautious.