Aside from meters, if You are healing the tanks who constantly take damage, and he is healing the raid witch avoided most of the avoidable damage, the fact that the raid
itself didn't took much damage made his "healing meters" look worse, so the raid being good may make your friend look bad...
And shields last for 15 seconds. unless it's an emergency shield, if he miscast on someone who doesn't needs how does the meters react? unused 20k shield equals to