Would a flying-wing drone fly farther if filled with helium?
If the end result of the sealed wing filled with helium is lighter than the same wing filled with a normal atmosphere (nitrogen, oxygen, and assorted other gases), then yes - a lighter aircraft will, all else being equal, have better performance and thus longer range than a heavier one.
If you'd actually be able to achieve economic benefit is doubtful - the delta in weight will be offset by everything required to seal the wing up, and the gain in efficiency may be pretty tiny. Beyond that, if mechanics need to access stuff inside the wing, you'll be paying to replenish the helium whenever that happens. Plus, the volume inside the flying wing is normally where things like fuel, passengers, crew, cargo, engines, and etc would go; you may find that you don't have all that much available space left to fill with helium.