The reason why relapses occur is because the PWS (person who stutters) starts taking fluent speech for granted if fluency has occurred for a week or two. Stuttering is a habit. The bad speaking habits are dominant in our brains and are thus automatic. This needs to be turned around so that fluent speaking habits eventually become dominant, and the bad speaking habits wither away. So I have realized that I have to continue doing my exercises every day, even when I think I don't have to. If I stop my exercises for a few days, I will relapse. I need to do my daily affirmations, project confidence, read out loud to myself where I am 99% fluent so my brain hears it and becomes accustomed to it, and practice my tools/crutches so that when anticipations of blocks appear, I can implement them in real life and get around it. Over time, it will probably take 8 months to a year, fluency will become mostly automatic, because I will have literally changed the way that my brain is wired. The guy you knew who said he was mostly fluent but relapsed, likely stopped working at it, so the new correct way of speaking never became automatic in his brain. That's just my educated guess. There is no quick fix to stuttering. If we want to be fluent we need to constantly work at it. It's not easy, and it might be painful at times, but it's possible with the right attitude. Good luck and best wishes!