You only have to fight the Shadow if you plan on having Coriander decide not to accept it and deny it outright. If she's already had the experience and the Shadow isn't bringing up anything new, that she can likely more easily accept it and gain a Persona. It may feel like a cop-out but it's honestly fine! There's no sense forcing the drama and all that c:

If nothing else, if anyone is with her then she can be a good example on what to do when confronting a Shadow, really.