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Hi, and welcome to the community!
You need to on his computer; the go to Edit>Preferences and get Spotify to index the folder with the .mp3s in so it will appear in local files. Then drag and drop those local files from the 'local files' tab into a new paylist.
Make sure your device is connected eitehr via USb or the same wifi network as the computer and is showing under 'Devices' in Spotify desktop. It should update with the playlist but all the songs will be greyed out.
You can then enable offline mode for that playlist on your iPad and provided it is connected to Spotify desktop the songs should be synced to your iPad and then you can play them offline.
And then yes you can uninstall Spotify from the computer as the songs are now effectively copied to your iPad in an encrypted form.
Please note others can't add your songs to their playlists as they are offline files and not on the Spotify servers.
Hope this helps.