How to secure French doors?

Hello!

I have a set of single handle French doors that lead from the living room to the backyard. I also happen to have a grandmother with dementia who lets my cat out if we’re not looking and he cries at the doors. Does anyone have any ideas on how to secure the doors from the inside? Not worried about fire safety necessarily- we’re usually home and if we’re not, we lock the cat in our bedroom.

I’ll attach a picture of the doors as soon as I can

Thanks for any advice!