A first guess would be that in the author's idea, Oxygen would be used to pressurize the helmet and meet the Astronaut's metabolic needs, and Helium to pressurize the spacesuit. That would be a terrible design though, at least concerning insulating properties : thermal conductivity of Helium is very high (152 mW/m/K) compared to Oxygen (23,4 mW/m/K) or some rare gases used in technical diving like Argon (17,7 mW/m/k). Conducting Extra-Vehicular activities in such conditions would be extremely demanding for the temperature control system which would be easily overtaxed.
As you raised it in your question I suppose that you are familiar with Lorrain-Smith hyperoxia-toxicity effect : an extended exposure to an oxygen partial pressure over 0,5 Bar leads to pulmonary inflammatory lesions. It is possible that the author, although he was familiar with this very specific effect, was not familiar with the possibility to conduct EVAs with a spacesuit pressurized under the atmospheric standard pressure. Although this is unlikely, this misconception could have led him to imagine the use of Helium to reduce ppO2 below 0,5 Bar in the spacesuit.
Here is my suggestion : pressurizing the suit with Helium is a way to get rid of the pre-breath protocol currently used by our Astronauts in ISS sas QUEST. Explanations :
- As you know, ISS has a fairly Earth-like, sea-level atmosphere: 21% Oxygen - 79% Nitrogen. During EVAs, the EMU or ORLAN suits of Astronauts are approximately pressurized at 1/3 of an atmosphere with 100% Oxygen. As they suit on and begin the EVA, the Nitrogen desaturation gradient would therefore be very high between their body saturated at 0,8 Bar of ppN2 and the 0 Bar ppN2 pure-Oxygen they breath.
- This is not a problem if the total pressure is high enough (you have no decompression sickness on earth if you begin to breath pure oxygen at the surface), but as soon as the spacesuit is only pressurized at 1/3 of an atmosphere, conditions are met for an explosive desaturation of Nitrogen. This is why Astronauts breath pure oxygen on the ISS for 2 hours and 20 minutes before the EVA, with short periods of high intensity exercises, in order to wash out Nitrogen before any exposure to a low pressure environment.
- That being said, you have two options to avoid decompression sickness during EVAs: either you perform this cumbersome pre-breath protocole, or you perform the EVA with a 1 Bar-pressurized spacesuit. For the second option, if you use pure oxygen, ppO2 will be more than 0,5 and you will be exposed to oxygen toxicity (Lorrain-Smith effect). You can use then either Nitrogen (the mix would then be close to air), Helium or other gas as a trade-off between a totale pressure high enough to avoid decompression sickness, and a ppO2 low enough to avoid toxicity.
- Why Helium ? You could use Argon as it is a much better insulator, but it is far too dense and can lead to asphyxiation. This lets us with Nitrogen or Helium. My opinion is that Helium is more future-like, sounds like technology and this is why the author chose it.
Please note that this is only a mental experiment: a spacesuit pressurized at 1 bar would lead to numerous other problems, for example the stiffness of the joints.