A friend just replaced my failing hard drive on my iMac 21.5. How do we reinstall Catalina and original apps?

Welcome to Apple Support Community.

See: How to reinstall macOS - Apple Support.

Also, you shouldn't replace the HD yourself as it is an unauthorized modification. You should contact Apple: Get Support.

Jack

Once the OS has been reinstalled you can either reinstall the apps and files from a backup disk using Time Machine to do the restoration - if the computer's backed up. If not, then the apps can be reinstalled by downloading them from their provider's web sites. Some of them may have to be reregistered to use them.

New versions of Apple native apps will be installed along with the OS; the old versions are likely unavailable. Other apps you'll need to go to the AppStore, or download them from the developer's website.

Suites requiring a license will require you to buy a new license (version) or attempt to re-activate them using the old license. That will require you install those licensed old apps in whatever manner prescribed by the developer. You'll need the installation media for the old licensed apps if they are even available, plus have the original license.

I hope your friend properly formatted the new drive before installation.