Keep in mind the 20GB of Free storage space mentioned by @neuroanatomist is referring to what is necessary just to boot macOS & perform the most basic tasks. Plus you need to ignore the "Available" storage value shown everywhere within macOS because it is very misleading. You need to look at the actual Free storage space value which unfortunately is only shown in Disk Utility or the System Profiler. With macOS "Available" is not synonymous with Free.
Plus for upgrading macOS to a new major version of macOS, you need a lot more Free storage space. In addition to the 20GB minimum of Free space, you need another 17GB just for the Tahoe installer. Then you need at least another 17GB because the installer needs to remove & expand the files contained within the installer app. That means you need at least 54GB of Free storage space before you start the install process. Hopefully you have even more Free space than that when performing the upgrade.
FYI, bad things will happen if you completely run out of Free storage space. With the APF file system it may be impossible to even delete any files to make more room due to how the APFS file system works.
Not much you can do if the other user has a ton of data they don't wish to relocate.
Besides, there is no immediate need to upgrade to Tahoe if you are currently running Sequoia.