I used to use a hairdresser where I could get a cheaper haircut if I had it done by one of the apprentices. 2 times out of 3, that worked out fine - once in a while I rather felt that they should have been the one paying me!
But that doesn't mean that the apprentice isn't still being paid apprentice wages.
And an FI(R) gets paid typically around £10/hr less than an FI, whilst the student doesn't pay any less. That £10 presumably is supposed to cover their supervision.
It's how it is I suppose, but flying for free is dodgy most places and none of us should tolerate it in a commercial environment. Fine amongst mates or family, but not otherwise.
G