First, it's his fault his marriage is falling apart, not yours. He knew what he was doing, he played a stupid game and won a stupid prize.

Second, it wasn't your place to tell the wife. You don't go out looking for a SD or any other arrangement, and then turn that person's life upside down. You're going to meet some really weird underdeveloped people doing what you're doing, that's a risk you have to take if you're going to do it. Kind of messed up that you knew he had a wife and kids, "didn't care" and did it anyways, and then pulled the rug out from under him. He had it coming, but it wasn't your place to tell.

Get into therapy if you're not already and figure out why you even wanted/needed a SD