The source matters because people accept too many things at face value. If you're given bad information and act on it, when that source lacks credibility, you accept the consequences for believing it. If you take stock tips from some guy screaming them on a street corner, you've pretty much assumed all responsibility for taking advice from what's unlikely to be a reliable source. You can't turn around and say 'Hey, you gave me bad advice, it's your fault I'm ruined' (well, you can, but don't expect anyone to take you seriously).

And maybe she did have an allergic reaction. That's an unforeseeable consequence (unless she knew she was allergic, but then it puts it back on her). It has nothing to do with any of the points I made.