This summary is not the rule. The rule text controls.
If someone files a lawsuit on behalf of another person or entity (like an agent, executor, or guardian), the court automatically assumes they have the legal right to do so. They don't need to prove this upfront.
However, the other side can challenge this. If the defendant questions whether the plaintiff really has the authority or legal standing to sue on someone else's behalf, they must raise this objection promptly through a specific legal filing called a "dilatory exception." This needs to happen early in the case, not whenever they feel like it.
If the defendant does raise this challenge, the burden shifts to the plaintiff. At a hearing on the matter, the plaintiff must then actually prove they have the right or qualification to bring the lawsuit on the other person's behalf. So while the law gives plaintiffs the benefit of the doubt at first, they still need to be ready to back up their claim if someone calls it into question.