Although Gary Sues like this in Dr. Who world aren't too bad. They make sense and make things interesting. And he doesn't have all of God's knowledge, only the past and only Earth's past. Which Jack is part of since he'd been to it and thus why he knows the man's real name.

If you ever need any help, toss me a mail. I have no problem at all with them. Figuring out what to do with them...now that's an issue.

Suicide? Angel? I could see him trying to epically fight them because he suspected they were the ones who did it first. In fact that could be the poly with convincing him she's dead. They could even admit they did it and lead him into a trap.

I personally think, and no idea if this has been used before or not, but I think Willow or Xander should convince her they have a soul to make her welcome then close to her and then kill her.