I don't necessarily disagree (or agree) with the calculus that we arguably could and should be injecting SO2 into the stratosphere right now as an emergency measure, but your acquaintances are certainly not the only ones thinking about it. As the first article I linked mentions, the idea has been around since at least 1965 and references a 2021 landmark report. The White House OSTP also released their own 44-page report on it in 2023 (https://www.cnbc.com/2023/06/30/white-house-releases-report-... , https://bidenwhitehouse.archives.gov/ostp/news-updates/2023/...), pursuant to the research plan from the 2022 article I initially shared.

I'd also probably agree that there is likely misguided opposition to it as a tool in the climate change arsenal as well from "climate advocates" (taboo). The same could also be said for fission nuclear power which, unlike SO2 geoengineering, would substantially address the root cause of the problem - emissions - with fewer risks and unknowns. (France, for example, being a real-world example of how many countries could almost completely decarbonize their electric generation in a proven and scalable way with nuclear fission.)

If we further broaden our scope of misguided opposition from just "climate advocates" to voting polities in countries that are positioned to meaningfully address climate change at a global scale, then we're really getting to the root of the issue. The single most impactful action the average person could take to fight climate change in the US is to vote blue. It's an effectively binary choice to give badly-needed societal support and investment to climate-relevant initiatives like your friends' and so many others.