Screen Showing Horizontal Black Lines

It's possibly a hardware issue, but there are some things to try:

Boot into Safe Mode and see if anything changes: Use safe mode on your Mac - Apple Support

If nothing has changed, you could run Apple Diagnostics on it: Use Apple Diagnostics to test your Mac - Apple Support

Please let us know if either of these tips was helpful.

While it's difficult to pinpoint the exact cause without more information or being able to examine the device directly, there are a few possible explanations for the horizontal black lines on the display. Here are a few suggestions to help troubleshoot the issue:

1. Restart the MacBook Air: Sometimes, software or temporary glitches can cause display issues. Ask your colleague to try restarting the MacBook Air to see if it resolves the problem.

2. Check for graphics card issues: The black lines could be related to a problem with the MacBook Air's graphics card or GPU. It's worth trying to reset the System Management Controller (SMC) to see if it helps.

3. Check for software-related issues: If the MacBook Air was running an outdated or incompatible operating system, it could potentially cause display abnormalities. If restarting the device doesn't resolve the issue, you could suggest booting into Safe Mode. Safe Mode disables unnecessary extensions and performs a basic check of the startup disk, which may help identify and resolve any software-related issues.

4. Inspect the display connections: It's possible that the issue is related to a loose or faulty display connection.

If none of these steps resolve the problem, I recommend contacting Apple Support or visiting an authorized Apple service provider for further assistance.