Perhaps to try & use the macOS Recovery instead of the old Snow Leopard DVD media
there may be better chance of resurrecting the 2012 MacBook Pro? That didn't ship with
Snow Leopard, so wouldn't hardly be able to boot or use disk utilities from it.
• About macOS Recovery on Intel-based Mac computers - Apple Support
https://support.apple.com/en-us/HT201314
..or Maybe it could've started into Safe mode?
• What to do before you sell, give away, or trade in your Mac - Apple Support
https://support.apple.com/en-us/HT201065
The last step 8, mentioned in above says:
"Follow the steps to erase your Mac and reinstall macOS.
After macOS installation is complete, your Mac restarts to a setup
assistant. To leave the Mac in an out-of-box state, press Command-Q to
shut down instead of continuing setup."
If not the above, to see if you can return Mac OS back to original system,
some kinda work around may do; to get the mac to run a newer one.
You'd have to look online at Apple support site, to get downloadable
installers for any supported older systems that may run on the Mac.
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
Good luck! ☺︎