This is a fascinating question! There's a lot more to it then first meets the eye!
First, to answer the boring question: Will this kill the hero?
Yes.
However, it won't be a very good story if the protagonist dies midway through, so as with any grand alien invasion, there's one aspect of Earth's environment that the aliens didn't count on, and it's going to be the Achilles Heel of their horrific weapon.
So, let's get started with this roller coaster of a science question:
There are multiple factors that come into play when dumping a solid, supercooled gas into a liquid substance, esspecially one as unique as liquit water!
1. Water is one of the only substances in the known universe that reduces in density when it freezes. This is the reason why lakes freeze from the top down, rather than bottom up. So regardless of how you cool a body of water, it will always freeze from the top down!
2. This counter-intuitive density behavior is made even weirder by the introduction of salt: Salt water has a lower freezing point then pure water and it also has a higher density. When you try to freeze sea water, this causes a natural desalination effect. As a result, the pure desalinated water rises as ice and the leftover highly-dense, sub-freezing brine shoots down in jets creating downward-growing ice spikes. Anything caught by these ice tentacles will freeze and die. Visual reference: YouTube: Brinicle, Underwater Icicle "Finger of Death"
3. Now this sounds pretty bad, but if the hero can evade the slow-moving ice tentacles, he still has a decent chance of surviving due to one really cool physics effect:
4. When you cool a gas to a liquid or solid state, its boundary layer boils very easily and turns to gas the moment it comes in contact with a warmer substance. This creates what's known as the Leidenfrost effect, where the boiling gas at the boundary layer forms a thermal barrier between the hot and cold substances. As a result the water won't freeze instantly! Here is a cool experiment where a guy tried to freeze seawater by pouring liquid nitrogen on it: YouTube: What Happens When You Freeze The Ocean? Pouring Liquid Nitrogen in the Ocean to Stop Global Warming
5. But wait! There's more! The Leidenfrost effect is imperfect. When the object is big, the huge amount of cold matter will still have a sufficient thermal transfer rate to freeze the surrounding water (though not instantly). In this case, you will have ice forming around the sphere, and that’s where the hero’s situation suddenly becomes very dire. 815
Ice and gas are good insulators, but not perfect, and the frozen helium inside will continue to slowly warm up. Eventually the increasing gas pressure will cause the ice shell to rupture...kinda like in this video: YouTube: Underwater Explosions - Slow Motion Dry Ice Bomb.
6. Luckily, the explosion won’t be big since the whole sphere can't be uniformly encased in ice (due to the non-laminar effects of real-world turbulence in water), so it won't kill the hero. Instead, the hero is going to suddenly find themselves inundated by hundreds of back-to-back concussive blasts as the small helium pockets throughout the ice expand and fracture their ice blocks. These micro-explosions will probably feel like gut punches, damaging internal organs and rupturing ear drums. (It’s going to be a bad day for anyone in the vicinity when this starts happening.)
7. If somehow the hero escapes the ice tentacles and manages to retain consciousness throughout the micro-explosions, their troubles aren’t over yet. Far from it. They will then need to swim up to the surface through a meat grinder. You see... all that ice we’ve been talking about, would have floated up to the surface and turned into the most dangerous form of ice you’ve ever seen: razor sharp ice sheets, crushing and grinding everything that gets caught in them (YouTube).
8. Suppose the hero somehow climbs through the ice flow and claws their way up on to one of the ice blocks. They miraculously get through with only crushed ribs, broken arms and legs, and a badly shattered pelvis, but otherwise they're totally fine! However... now they will face their final two obstacles:
9. First, all of the helium gas boiling off the sphere would have expelled most of the breathable air from the area, and the hero will probably suffocate.
10. Second, if there is enough oxygen left for them to survive, the air will be really cold. This will cause edema, and (sorry for the graphic content) they will literally drown in their own bodily fluids as their longs fill up with water. 815