What's a student?
http://www.ins.gov/graphics/glossary4.htm#S
Student - As a nonimmigrant class of admission, an alien coming temporarily to the United States to pursue a full course of study in an approved program in either an academic (college, university, seminary, conservatory, academic high school, elementary school, other institution, or language training program) or a vocational or other recognized nonacademic institution.
Part 61 flight schools do not fit into any of the above. Don't believe what people who run 'approved' schools tell you, it's in their interest to scare foreign students into coming to them.
Now the INS Inspector can deny entry to anybody coming into the US. Your buddy must have said the wrong things to a guy who didn't fully understand the rules, but the INS inspector was quite within his rights to refuse entry.
INS inspectors get it wrong you know. The US embassy in London will also err on the side of caution, which is sensible for them to do. Have you ever got off the phone with somebody who obstinately insists on giving you the 'safe' answer, when you know things in practice work differently? What if the Embassy tells you to go over on a visa waiver and the INS inspector refuses entry at his whim, you are going to be pissed and that is not what the embassy wants.
Don't get me wrong. If you at all worried then you should get the M1 visa and tie yourself to a specific school. Its not neccessary though in US immigration law if you can enter under the visa waiver scheme, which is my point.
I am not a US immigration attorney and not licenced to practice law, I got this information from my brother who is. He has done work for US flight schools and knows the law and a lot of INS guys.
Regards