I don't know, but somehow I think deliberately tricking people into doing things that directly cause physical harm to themselves falls on the illegal side of that line.
I'm quite sure the girl was not deliberately wanting his ex to die. The "I'll kill myself if you leave me" kind of emotional blackmailing is just too common among hurt exes.
Good question, but I think it's safe to take one of the largest Sharpies at OfficeMax and draw a solid black line right before telling autistic kids to drink bleach.