California Labor Law Question (Vacation/Comp time)

I have a question regarding a co-worker. She is a permanent employee at my company (in California) but is NOT salaried. Still, she gets vacation time and used one this past Monday. She worked 4 full days Tuesday through Friday, but is required to come in on Saturday for a time-sensitive event.

HR at our company said that she would not be paid overtime for this because she was under 40 hours worked for the week. She also would not be reimbursed for her vacation time or given that credit back.

Is this correct? It seems that if this were the case, employers could dick over their employees but forcing them to work weekends after vacations, since they wouldn’t have to pay them anything extra OR give them back their vacation time. Can anyone shed some light on the situation?

Never mind, answered my own question with some Google-Fu. Mods, you can delete this thread.