Transfer old MacBook Air data to new user account

You can use Migration Assistant as long as you only transfer your partners home user folder.....deselect everything outside of that specific home user folder or it will disrupt your current apps & settings.

Transfer to a new Mac with Migration Assistant - Apple Support

You should definitely take @D.I. Johnson's advice seriously and make sure you have a good backup of your new Mac because if something goes wrong during migration, then you will need to erase the system & start over again. You should already have a good backup of all your older devices as well. Using a backup can make the migration go more quickly as well.

Backup the old data to an external drive and copy it over to the new one.

You can do a straight drag and drop copy by means of the Finder, you can use a cloning app such as Carbon Copy Cloner or Super Duper!, or you can use Time Machine to back up their data and migrate it from the external drive to the newer Mac.

Whatever you decide, I strongly recommend that you make backing up the computer a high priority as you both continue to use your computers moving forward. With no backup strategy in place, your important data is but a single bad day away from being gone forever.

Your strategy should include the 3-2-1 technique; 3 copies of the data using 2 different means with 1 copy kept off-site for security.