Locked disk from purchase

Alright so, My mom just bought a computer from a lady on facebook, its a 2019 macbook air. The computer works fine, even looks great and runs great. But every so often a box saying "Enter a password to unlock the disk ""Macintosh HD - Data"". She enters in her password, wrong. she tries every password she knows and its wrong. I told her to go to disk utility and check the disks. She shows me there is the disk she's used, which is about 15 GB, and then this other one that shows its not mounted but is taking up 50 GB. Upon trying to mount, delete, or even run first aid on it, all it says is the same password popup. My question is, is this the old owners disk that is locked from file-vault, and if so do we just need the password for it? ( We were in Recovery mode while attempting to do all this disk utility stuff)