I erased hard drive. Have no internet connect to reinstall Mac OS Mojave. Restarting yields on large question mark. I want to give this MacBook Air to a friend. How do I reinstall Mac OS?

If you click on the link I gave you there should be different options for restoring an OS to your mac,

from reinstall the original OS that your mac came with or reinstall the last OS your mac was running.

This is an article from Apple, What to do before you sell, give away, or trade in your Mac - Apple Support

Apple recommends that you restore your mac to the original OS it came preinstalled with.

Now for the next bit when the Recovery Servers give error messages.

The certificates for several of Apples OS's expired in October 2019, Apple have been very poor at getting these updated on their Recovery Servers with valid certificates. This may be the problem you are experiencing. 

Try this workaround, we are going to set the time and date on your mac to a date previous to the expiry date of the certificates.

Connect your mac to your router via cable, not WiFi

Boot to your Recovery HD, click on Utilities in the menubar select Terminal.

Make sure WiFi is switched off, it can reset the date back to today.

Enter a new date, for example or just copy and paste

sudo date -u 011421002016

press Return

enter your password

press Return

If Terminal returns an error saying sudo : command not found, then try again without sudo.

Now try downloading the OS.

If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time

and reset the time back to today.

This is why it is so important for you to have your own copy of the OS to hand to stop you going through all this nonsense.

Follow these instructions: How to reinstall macOS from macOS Recovery - Apple Support.

Can I get OS Mojave installed without internet?

Yes, you can boot a Time Machine backup disk, which contains a Recovery partition. No Internet connection is required.

Only if you have got a copy of the Mojave installer to hand.

If you have gone ahead and wiped your mac without having a strategy to reinstall

the OS is a bit remiss.

You really need an internet connection so you can reinstall an OS from Apples Recovery Servers.

Do you not have internet access at home, or does your friend, or can you go somewhere

where you can access the internet.

How to reinstall macOS from macOS Recovery – Apple Support