I use avoidance strategies too. It's not that I do this constantly, but everytime when I am under pressure it becomes more natural to substitute words. Maybe I judge my friend to much, because I am more aware of what he is doing, than a fluent person is. But there are long pauses before every word he substitutes. There is another thing I experienced. I have a few close friends and my family that I talked to about my stuttering. As for instance, I requested them to not finish my sentences. After that, my stuttering decreased in an amount even they would notice. (reading and writing in english are two different things - feel free to correct me)