"Dependent on ATP levels, and hence blood glucose levels, the ATP-controlled potassium channels (K+) close and the cell membrane depolarizes
On depolarization, voltage controlled calcium channels (Ca2+) open and calcium flows into the cells.
Significantly increased amounts of calcium in the cells causes release of previously synthesized insulin."
"The glucose-alanine cycle is the main pathway by which amino groups from muscle amino acids are transported to the liver for conversion to glucose...
Plasma amino-acid levels are affected by dietary carbohydrate through the action of insulin, which lowers plasma amino-acid levels (particularly the branched-chain amino acids) by promoting their entry into the muscle.
Your friend is right about protein EVENTUALLY raising insulin levels, but digesting protein will not 'spike' insulin, it will instead create 'small waves' of insulin. All lifters, bodybuilders and powerlifters alike, want to 'spike' insulin at certain times of the day, such as in the morning and after a workout to maximize 'channelling' of nutrients into the muscles.
Remember, the rate of nutrients channelled over time is directly correlated to insulin concentration levels, so the higher the insulin the better the effect. This is why we want to spike insulin, and not just eat protein all the time.
Also remember there are some times when you do not want spiked insulin, so too many carbs can be bad thing. I always say carbs should be used like a tool, not a primary source of nourishment.