This exact situation happened to me. I booked lessons for my wife and I, she was a complete novice and I'd been shooting clays for about 3-4 years. I was getting late 30's/50 at Sporting (with the occasional 40+) fairly consistently, Skeet I was averaging 23/25 and DTL 22/25. For me I just wanted a few pointers about stance and some general advice. The lesson was on a Skeet layout and the instructor did not like my stance at all, so he advised I adjusted several aspects; weight distribution, body forward etc.
My shooting on the day was terrible. Next session at my local ground I shot 19/50, the following week 18/50. Skeet scores also plummeted. This kept on for about 3 months before one of the guys I shoot regularly with pulled me to one side and asked me to go through what I was doing when I entered the cage. I was thinking too much about my stance, when the clay was launched I was still thinking about stance, and then immediately switching to work out lead instead of relying on instincts etc.. Throughout the whole process my body was rigid, I simply wasn't relaxed and as a result not enjoying what I was doing which in itself is counter productive.
I went back into the cage (with the blessing of the rest of the squad) for about 20 rounds during which I adopted my old "natural" (for me!) stance, and simply tried to relax and enjoy what I was doing. After that my scores started to climb back up again (it took a few weeks).
I don't necessarily think the advice I received was bad, I just don't think it suited me or my style; certainly on the day my wife shot really well with the same advice. My problem was that I thought about it far too much and thought I was doing something wrong that needed to be corrected (which I probably am, but I can live with it!).