I think Leonard - the friend - is Desmond's replacement, and the reason he never showed up is that he ended up in the psych ward. Either Demond's replacement or the guy Desmond replaced.

In season one, Hurley sought out a friend of Leonard who also heard the numbers (radio listening station?), and found his widow. I don't think either were from the island, but picked up the transmission. One of them ended up dead, the other in a mental institution.

Seems those two made the same mistake Hurley did, which was "using" the numbers, and may share their fate. If anything, Hurley may be who Desmond is waiting for...