The issue with Lightforged Draenei becoming Warlocks is two fold. It has been biologically shown in the Warcraft universe that Draenei “devolve” when exposed to fel. The second issue is more of a cultural one in that this whole race exists just to destroy the Burning Legion which encompasses fel and demons. They even have a racial, Demonbane, that gives them more experience for killing demons. The best and easiest solution is to not have Warlock as an available class to this race. Since I have doubts the best option will be implemented, let me propose an alternative in building the lore for supporting it.
Firstly, this combination should be locked behind a quest that explains how a Lightforged Draenei could be a Warlock. This would ensure the player has a coherent understanding of why this unthinkable combination could be allowed.
The unlock quest should contain some background that allows the Lightforged Draenei to be shielded from fel magic. Perhaps the Lightforging process in itself protects them? This would keep the Lightforged Draenei from devolving. Additionally, the demons that are summoned should be lightforged as well to show that perhaps they have converted to the light.
I’m open to any type of background, this way just seemed like the most straightforward.