I believe he is running 11 since he's running beta. Photos app problems has been an on again off again thing for some people ever since 11 came out. Here are some things to try, checking along the way to see if photos works.
1. run the app troubleshooter- settings-system-troubleshoot-other troubleshooters-windows store apps-run
2. reset the app - Settings-Apps-Apps & Features-Microsoft Photos. Under Reset click 'repair' If a repair doesn't help the problem, then click 'reset'.
3. Uninstall the app - In Powershell, run the command get-appxpackage *Microsoft.Windows.Photos* | remove-appxpackage to uninstall Photos.
Be sure to restart here.
5. Go to the Microsoft Store and install Photos again.
6. Check for file system corruption. Open command prompt as administrator and type sfc /scannow. Report back if system file checker finds any corruption it is unable to fix.