You would probably just use pseudostuttering (fake stuttering like a non-stuttering actor would do) and use your past knowledge of how stuttering sounds and feels to portray it better than a non-stuttering actor. Also, there are more than the classic repetition types of stuttering (b-b-b-banana) that non-stutters and non-SLPs don't know about (blocking and elongation) so the person you were talking about might have that or it just comes and goes (unless he flat out said he doesn't stutter anymore). Most likely you would not try to "unlearn" it. A lot of people still identify as a stutterer even if it "goes away" with speech therapy as it can often come and go and the effects it has on one's emotions are still there. I can have an emotionally bad day in regards to my stutter even if others don't hear it as much. Good question! edit- changing stutter to stutterer, I always do that