Try restoring the phone using iTunes. You will have to restore as new.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
If that fails, try putting the phone in recovery mode.
If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support
Here are a couple of sites explaining how to remove a jailbreak.
https://www.macworld.co.uk/how-to/iphone/unjailbreak-your-iphone-3601679/
https://appletoolbox.com/2016/03/factory-reset-jailbroken-iphone-ipad/
They're both a bit old, and since I've never jailbroken any of my iOS devices, I can't vouch for how well the steps will work. Particularly on a newer iPhone 8.
The general steps are to first use iTunes to backup the phone so you can later restore your contacts and other personal data. Then follow the outlined steps to restore the iPhone to factory condition.
In a jailbroken state, Apple will not for any reason provide warranty work, even if the device is still within the warranty period. Once you reset it, the phone will be the same as if you took it out of the box. The warranty should then still be in effect.
Who knows how he did it. Ask him. But once done, the warranty etc is void and it cannot be undone.
Oh my gosh
Ah! Thanks, Demo. I knew Apple had a virtually identical page of instructions, but I couldn't get it to appear in a search.
Jailbreak
