Let's assume your roommate is right for a moment and think about things from his perspective. For him, the case for chemtrails is straightforward, because he can see the particles. They're right there in front of his face. Other people say they can't see anything and that it's impossible for him to, but those people must be wrong based on the obvious evidence of his senses.
So, there's a weird coating on everybody that nobody can see (or so they claim). What is it? He goes online and eventually finds descriptions of the chemtrails "conspiracy theory". The theory exactly matches the evidence of his senses. It explains why the particles seem to descend from the sky, why they coincide with the appearance of so-called exhaust trails, and why they suddenly started appearing in 2009 - all things he's observed directly.
The scientific evidence is naturally limited because this is a government conspiracy. Even the government can't completely prevent information from spreading on the internet, but they can absolutely exert control over institutional sources of knowledge like academic research. Thus, the theory's only proofs are implicit in tangentially related pieces of research, and its only advocates are forced out of the establishment onto public platforms like YouTube.
Anyway, your roommate doesn't fully understand the scientific component. He's not the smartest person in the world, he knows that, so he's a little insecure about his intelligence. He's just one man, so he can't really do anything about the conspiracy other than tell the people he cares for about it. He tells his fiance about it, and she seems to get it - she tells him she's spreading the word too. (She's lying, but he doesn't know that. He's exposing a government conspiracy, so maybe she's a plant. More likely she's just humoring him.) He tells his roommate, but he can't seem to get the point across. He doesn't understand why, so sometimes he gets frustrated.
Is any part of this account inconsistent with what you've observed? Isn't what you've seen exactly what it would look like if your roommate, an ordinary man, had a posteriori knowledge of chemtrails?
If this happened to you, would you act any differently?