Then she can be there to show support when the rest of the pride learns what happened.
Perhaps Simba confides the “truth” to Nala and she’s the one who pushes him to go back anyway and face his past.
The film could do this in a number of different ways, but the most important element is that somebody, Nala, Sarabi, anybody, needs to understand how difficult what Simba is doing is and support him doing it.
Simba might even throw out some statement of willingness to face punishment, the punishment that won’t be necessary when it comes out that Simba isn’t responsible of course, but dealing with consequences is also important when facing up to your mistakes.