It's worth noting that hydrogen is orders of magnitude more energy-dense, by mass, than lithium-ion batteries.
As a benchmark: the DJI Phantom weighs roughly 1kg, and its battery can store about 80kJ of electrical energy. I found a NASA study [1] which estimated that the energy released when a li-ion battery combusts is roughly 2x the usable energy capacity. So let's say that a battery fire would release 160kJ of energy.
In contrast, a balloon capable of lifting a 1kg drone would need about 830 liters of hydrogen, with a mass of 75 grams. That amount of hydrogen, if burned in room-temperature air, would release about 9MJ -- roughly 50x as much energy as a battery failure.
(Of course, most of the hydrogen would burn itself up quickly and harmlessly, assuming the balloon isn't flying near any flammable objects.)
Plus assuming the drone is above the tree line and not next to a tall structure, the actual "fireball" should move fairly skyward and not pose much of a fire threat? I'd be more concerned about the falling drone/battery.
It's not only direct contact with the fireball you have to worry about, a large amount of the energy in the explosion would be coupled into a pressure wave and infrared radiation.
