Legal Question in Real Estate Law in California

I'd have to say "no" they can't.

You can file a demurrer on the ground that the plaintiff lacks the legal capacity to sue.

Mr. McCormick may be right, but I wouldn't be so hasty to conclude.

First, a DBA, or more correctly, a fictitious business name, is supposed to be filed and published in the county of the firm's principal place of business. You say one hasn't been filed in any county, but from my experience I know checking all 58 California counties would be a big chore, since many of the counties don't have on-line access to their fictitious business name registrations. I wonder if you have checked them all.

Next, a foreign corporation that has not qualified to do business in California is not barred from initiating a lawsuit here. Corporations Code 2203(c). Failure to qualify is a defense, if the defendant can show the foreign corporation transacted intrastate business here before qualifying, but it does not bar the suit. United Medical Management, Ltd. v. Gatto (1996) 49 Cal.App.4th 1732, 1739-1740.

The suit itself may state the type of entity, its domicile, and any claim it has to be duly qualified to transact business. Many plaintiffs using DBAs will also state that they have duly registered their names. If I knew what the complaint alleges in the way of jurisdiction, venue, qualification, standing and the basis of the claims for right to possession, perhaps I could advise you further.

In any event, even if the plaintiff is not qualified, the lack of qualification or standing may only be a defense that must be raised after you file a responsive pleading, rather than a factor that deprives the court of jurisdiction, so be cautious and file a response.