I accidentally deleted Facebook from my husband's iPad and can't figure out how to get it back

Go to the App Store App. Tap Search button on lower right hand corner. Enter Facebook in the search bar. Tap the cloud icon next to it and it will reinstall. He may need to sign back in.

Re-downloading the app from the app store would be the usual path.

I’d leave it off entirely, and let your spouse decide to re-install it.