One of the main issues I see with this is that people lie.

They lie so much, that almost everyone in HR completely discounts employee based salary data. Instead, if they need to understand what a market rate is for a salary, they ask employers what they are paying, since to them it is just another expense.

Talk to anyone worth anything in HR and they will echo exactly what I said. You may as well guess what your buddy is being paid, because whatever they tell you is never going to be accurate enough to provide you with an informed opinion.