After upgrading to Windows 10, can I give my Windows 7 serial key to my friend?

I own a legitimate retail version of Windows 7 serial key. I upgraded my computer to Windows 10.

My friend built a new computer and he has no Windows license. Since I won't use Windows 7 anymore, can I give my license to him, while keeping my free Windows 10?

Your Windows 7 license won't become invalid by updating to Windows 10. However, it is also part of your Windows 10 license, so you are only allowed to have one Windows 10 or one Windows 7 installation at a time with one key.

When I upgrade a preinstalled (OEM) or retail version of Windows 7 or Windows 8/8.1 license to Windows 10, does that license remain OEM or become a retail license?

If you upgrade from a OEM or retail version of Windows 7 or Windows 8/8.1 to the free Windows 10 upgrade this summer, the license is consumed into it. Because the free upgrade is derived from the base qualifying license, Windows 10 will carry that licensing too.

Your Windows 7 license will always be valid and will not be changed or deactivated because of the upgrade to Windows 10: you'll be able to install or restore Windows 7 again in case you'll need to do that (provided that you've the Windows 7 installation DVD

Source