I used my Apple ID to sign in only to FaceTime on my friend's iPad device. Later I went to Settings → FaceTime and clicked Sign out. Does that mean that this iPad an my apple ID are now completely disconnected? Or is there a chance that my friend can get my FaceTime calls or even my iMessages etc.?
Yes - tapping the sign out button disconnects FaceTime. The messages app uses a different account unless you set them all up using iCloud preference or a setup assistant screen to log into all icloud services with one Apple ID.
As long as you signed out of the FaceTime portion of the settings app - more deliveries won't happen to that device.
as long as you didn't sign in to the iCloud settings or anywhere else - you should be safe with no data of yours on that device other than for the moments you allowed it.