As long as the statute of limitations hasn't run, the plaintiff can just re-file the case in Riverside, since the LA court's dismissal will be without prejudice to the right to re-file. It makes sense for you to follow through with this if you don't want the case venued in LA.
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The plaintiff can refile in Riverside, but you should get the case dismissed anyway, and seek costs for having had to respond to a complaint filed in a facially improper venue.
If plaintiff refiles in Riverside, you will obviously have to respond when served, but you should check if there are statute of limitations issues that would bar plaintiff's new filing, because the calculation of time will not relate back to the originally filed and dismissed case (whereas it would if it were transferred).
the short answer is that he can refile unless the statute of limitations has run.
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