I'm saying this because I recently watched a scene where a man gives another man his helmet because his cracked, they had trouble breathing the atmosphere and from my own research it seems they both would have likely gotten high. From what I can tell the oxygen level of the planet is maybe around 10 to 15 percent? given that they didn't imminently lose consciousness which has been reported (from a document i looked up) to happen in these conditions when the oxygen level is 10 percent or lower.
It's entirely an asphyxiation risk, not a toxic issue.
Just for fun...Argon has the opposite effect of Helium...speaking with high amounts of Argon in your lungs produces a much deeper tone than Helium. Expect these people to speak in very low tones, Barry White has nothing on these guys
Other answers touch on it, but full issue is - Argon is heavy. It sinks in the atmosphere and finds lows to gather in. If your characters are on a hill or in a windy environment, this might not be as much of an issue. If they are in a valley, the argon will pool, replacing all oxygen, and cause death. Ugly too, breathing fine, but dying from lack of oxygen anyway.
Argon will also pool in your lungs. Prolonged breathing of it will see Argon start to gather in the lungs and not be easily replaced, once again becoming an asphyxiation risk. Even if there is ample oxygen in the air, the argon prohibits it from getting to the lungs. Causes short rapid breathing before asphyxiation. This is actually why Helium voice is generally ok while Argon voicing is considered highly dangerous.
Remember, the oxygen requirement we have is based in pressure, not simply percents. We require around .2 Atm of oxygen for our respiration. Maybe a different question, but 10% oxygen in double the pressure environment could see us breathing safely. I'd have to research the topic further
Humans have always lived in a primarily nitrogen atmosphere.
To quote Wikipedia
By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide.
Argon is non-toxic. As long as the oxygen content remains around 20% you can have as much argon in the air as you want.
Argon is a noble gas; its chemical reactivity is so low you can effectively treat it as "filler" to keep the pressure up. Likewise, the bonds that make up diatomic nitrogen are so stable that they don't play any role in human respiration. (Certain bacteria can fix nitrogen, but that's out of scope for this discussion.) Those two gases make up 80% of our atmosphere, and other than relative mass they contribute to planetary air pressure, I see no reason why their ratio would be significant in breathing.
One thing we haven't considered, however, is air pressure. As pressure goes up, there is a risk of nitrogen narcosis, where atmospheric nitrogen gets into your blood and affects your brain function. This is why deep-sea divers use helium in their mix. That stack exchange article also mentions that the percentage of oxygen you need goes down as well when operating at a higher pressure; so increasing this pressure would increase risk of narcosis while reducing the need for 20% oxygen in your mix.
Even at "normal" pressures, however, SCUBA diving is probably a good metaphor for the story you describe. In their case, most of the air has been displaced by water, which we also can't breathe. In the old days of SCUBA, divers were trained to buddy breathe, sharing a single regulator between them in case of an equipment failure. This sounds a lot like the fictional story you describe, where the two characters are buddy breathing with a single helmet.