No Kingwolfy and your opponent is all wrong...Your opponent is right.You must have summoned Horus before you can use it after is sent to grave.If its discarded it does not work like that..here are rulings from netrep
An “LV” monster that normally cannot be Special Summoned except by the
method in its text CANNOT be Special Summoned from the Graveyard with
“Level Modulation” unless it has been previously Special Summoned by
the correct procedure. The “ignore the Summoning conditions” text
allows for these monsters to be Special Summoned only after they have
been properly Summoned by their own effect. This applies to “Horus the
Black Flame Dragon LV8”, “Armed Dragon LV7”, “Silent Swordsman LV7”,
“Silent Magician LV8”, and “Winged Kuriboh LV10”.
All that means Horus must have hit the field before destoyed in order to use the spell card..so your opponent was right