First off, he didn't. Faith shouldn't have found out about Angel's return from Xander, but she was the one who jumped to the idea of killing Angel. And while Xander was initially supportive, later he was the one arguing the Angel wasn't the person who knocked out Giles - and when Faith went off to kill Angel, it was despite Xander telling her to wait.
But even if he had manipulated her, there are deeper problems with blaming Xander for Faith's turn to darkness. Despite the fuss about Allen Finch's death, that was not Faith's first evil act. Faith's first truly evil act was her attempted rape/murder of Xander. To blame Xander for Faith's turn to evil, then, is what we call "victim blaming".
And at the end of the day, Xander was a kid. I've said this with Buffy wrt Faith, and it applies to Xander, too: Xander was Faith's peer, not her parent. He was not responsible for her, anymore than she was responsible for him - and when he did try to help her, he was viciously attacked. Xander can't be held responsible for Faith's choices.