TeamViewer allows you to set a fixed password, precisely for these sorts of situations.
In your friend's TeamViewer settings:
- Enter a strong password that you don't use anywhere else
- Choose random password: "Disabled (no random password)"
TeamViewer always needs a password for security reasons but the password doesn't need to change every time you connect.
You can also add his computer using the "Add remote computer" option in your TeamViewer settings, so that your TeamViewer remembers both your friend's TeamViewer ID (which won't change unless he gets a new computer) as well as his TeamViewer password (which is now fixed):
You will soon be able to instantly connect to your friend's PC simply by double clicking on an icon in your TeamViewer, without typing anything!