if the water damaged mac doesn’t turn on, the target made with a thunderbolt cable won’t work. You will have to take out the SSD. For your Mac, you will need a specific drive enclosure to match the SSD. You will also need 2 screwdrivers, a torx5 and a pentalobe for opening the bottom case. Check out owc/max sales. They have the enclosure and the screwdriver kit.

https://eshop.macsales.com/item/OWC/MAU3ENVOYPRO/

https://eshop.macsales.com/item/OWC/TOOLKITMBA/

If the drive is encrypted, when it mounts on the functioning mac, you will get a prompt for the user name and password for the main account of your partner’s laptop.

I don't know all the specifics about Macs, but I've done a little Googling and have replaced storage devices in PCs many times, so here's my 2 cents:

• It seems like the HDD is just using SATA and is 2.5", so an enclosure for that would work, but I would NOT store things you care about on a 10 year old HDD with potential liquid damage. Just get a SATA to USB adapter to get what you need.

• If there are 2 HDDs, chances are only one of them has the personal data you care about on it.

• I don't think it would be particularly difficult to open and remove the HDD, assuming you have appropriate tools.

• Linux may recognize the HDD. Otherwise, macOS will work.

EDIT: It sounds liku you may have the retina display version, which came with an NVMe m.2 SSD, which is not SATA.

Edit: looks like it's a 2012 retina MBP, which means it has an SSD with property connectors. Check out this ifix guide on how to disassemble them and what enclosure you can use to mount the drive in another computer: https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Late+2012+SSD+Replacement/12823

Those 2012 MBPs are using standard 2.5" SATA drives, so you can use any USB3 SATA adapter that supports 2.5" drives. Remember, always use USB3 sata adapter. Somehow there are still USB 2.0 sata adapters sold online right now, probably for legacy stuff so be sure to not accidentally buy them.

The drive itself is usually formatted in APFS partition, so you'll need to connect the drive to another mac in order to use. If it's encrypted, the encryption key is probably stored in the apple account associated with the broken macbook.

Your main concern is probably whether the disk got water damage or not. If the disk is damaged and unreadable, you'll probably can still recover it by paying a data recovery specialist in your area. They're expensive but can literally revives the drive as long as the platters are not damaged.

Good luck!