Stan will not notice he is suffocating. He will pass out and die.
Our need to breathe is triggered by accumulation of CO2. High levels of CO2, even with adequate oxygen, will provoke the sensation of shortness of breath and suffocation.
In contrast (for normal healthy people like Stan), if there is not high CO2 you feel no particular need to breathe from progressively lower levels of O2. When your O2 gets low enough you just pass out. This is called shallow water blackout.
Shallow water blackout which occurs when all phases of the dive have taken place in
shallow water (i.e., where depressurisation is not a significant
factor) and typically involves dynamic apnea distance swimmers,
usually in a swimming pool. The mechanism for this type of shallow
water blackout is hypoxia expedited by hypocapnia caused by voluntary
hyperventilation before the dive. Blackouts which occur in swimming
pools are probably driven only by excessive hyperventilation, with no
significant influence of pressure change. This can also be
described as constant pressure blackout or isobaric blackout.
This happened to me once from breathing helium to make my voice high. I blew out my air and breathed a lungful of helium and talked squeaky. When it was gone I took another breath of helium. Then I came to under a table.
In this circumstance, Stan has argon instead of helium. Argon does fine clearing CO2 out of the lungs and so he will not notice anything wrong. It is a pretty good defense for those monsters.