For all part-time, intermittent, reduced hours, and reduced wage situations, the Employment Development Department (EDD) suggests that you file a claim, and after we receive your properly completed claim, we will determine if you are eligible for Disability Insurance (DI) or Paid Family Leave (PFL) benefits.
If you return to work on a part-time basis because you are unable to perform your regular or customary work, and have a continued wage loss due to your disability or family care, you may be eligible to receive continued DI or PFL benefits.
https://www.edd.ca.gov/disability/Part-time_Intermittent_Reduced_Work_Schedule.htmThis is arguably not the exact situation you describe, but the general principle seems to be that you can collect benefits even if you're still working, if you're getting less money.
In a comment, you say:
I'm just having trouble figuring out the rules and if there's a point to file for it at all, or if they'll be rejected since they still have income or not, and trying to figure our what kind of income they need to plan for.
For the question "Will the application be accepted", I would say "Probably", but for "Should I submit an application and see what happens", I'd say "Sure".