Although it’s not what Syncthing was intended for, a Send Only folder type plus the ignoreDelete setting could be used with the caveat that there’s a chance that a file could be deleted before it’s been successfully synced from your mom’s PC to your dad’s PC.
Also, configuring Syncthing to accomplish the use-case described above would mean the sync status would always appear as out-of-sync because your dad’s PC has files that your mom’s PC says should have been deleted.
Since your dad’s PC is providing SMB access, a much simpler method would be for your mom to just move less frequently used files as needed off her laptop to the SMB share. It’ll feel more natural because it’s just another drive letter (or mounted folder) on her laptop versus a more complicated combination of Syncthing and file deletions.
I think we should really stop suggesting this as an option.
It’s hidden and not visible/available for a reason, and we should not encourage new users to use unsupported workflows, that we know will lead to poor experiences.
If the software does not support the indended users workflow, the user should seek alternative software that is made for purpose of what they are trying to do.