Can he still use Return of the Dragon Lords to resurrect it from the Graveyard?
Horus lvl 8 specifically indicates:
This card cannot be special summoned except by the effect of "Horus the Black Flame Dragon LV 6".
So the only way you can special summon it is with Horus lvl 6; once dead you can't bring it back with other cards.
On a further note, there are some cards that you can bring back after they are destroyed. If you see any card that says "Must be first special summoned by...", then those cards you can bring back after they die, but only if the first time you special summoned them was as the card specifies. After that you can repeatedly special summon it by other means.
One example is the famous Black Luster Soldier - Envoy of the Beginning:
Cannot be Normal Summoned/Set. Must first be Special Summoned (from your hand) by banishing 1 LIGHT and 1 DARK monster from your Graveyard. [...]
This means you must first banish one light and dark monsters to summon BLS, but after that successful summon you can special summon it by other means (like Call of the Haunted).
Edit: As per @Kitsunezeta suggestion, one card that your friend could use to do ignore that special summon condition is [Level Up!]:
Send 1 face-up "LV" monster you control to the Graveyard; Special Summon 1 monster from your hand or Deck that is listed in the sent monster's text, ignoring its Summoning conditions.
Of course this enables to pull Horus from the deck or hand, so still if it's destroyed its gone for good.