I may of course be missreading things, but what I gathered is that the ex is writing insane ravings on the site (using your name placeholder) sallysmith.com, not about Sally Smith, but implicitly AS Sally Smith, with the intent of making it look like Sally Smith is a raving nutjob to those who do a search for her name. Presumably, it's a rather more unique name than Sally Smith, so there is less probability of it being ignored as a naming collision. However, as the OP's fiancee is apparently a writer, it could be written in a way to make it really look like the author of the books and the author of the crazy screed are the same person without actually claiming they are.
If the site DOES claim to be the author, the fiancee may have grounds for slander of title, or, in some jurisdictions (and heavily dependent on the content of the crazed ramblings), there may be infringement on moral rights related to the original works.