When your friend’s laptop is at full disk, stop them from trying to make space by going through their tiny documents and deleting things that will be missed or finding that pesky Library folder that contains a bunch of gunk they never wanted and deleting that.
Go to the Mac App Store and download Free Disk Space. It will show what is really taking up all the space and should be dealt with.
Personally, I’ve used Space Gremlin for years, but Free Disk Space is free and does the job.
Update: Apparently Free Disk Space decided money is good. They are $10 now,