[ca]please Help I Don’t Know What To Do!

My mom and dad have rented an apartment in Los Angeles for about 30 years but my dad passed away 6 months ago. My mom has been paying rent like normal for the last 6 months. Today I learned that she isn’t on the newest lease, she was on the original lease made in the 80s when they first started renting the unit but the building has switched owner throughout the years and this last time they switched they made a new lease and my mom isn’t on there. Is this a problem can they kick my mom out of her apartment? Is there a way to get her on the lease? If so what is the best way of going through with it?