A few things!
I went to Bucknell in the early 2000s (I am currently 44) which I'm sure everyone who is familiar, is aware that plenty of people did coke recreationally there. It was not viewed as a big deal.
Of those people, a portion of them continued to use coke after college- mostly the crowd that went to NYC to work in finance. The other portion who went on to grad school, or a different field of work besides finance, all pretty much stopped.
Of the people who continued after college- as far as I can tell, most of them tapered off and stopped by their mid 20s.
For the people who did not taper off and stop by their mid 20s, one of a few things happened. One of those people is now in jail, due to massive psychiatric issues that he didn't have in college. I don't know if the drugs led to them or not, but, I am quite sure they didn't help. Four of them are dead- three of classic overdoses, within the past few years, and one of a heart attack/ heart problem of some kid, that person had an on and off eating disorder along with her coke habit so it's likely they both contributed.
Why did so many of them die back to back in their mid to late 30s? Was it the cumulative effect? Was it that they kept taking more and more, and had moved onto harder drugs that I didn't know about? (less likely, IMO, since they continued working and being outwardly "successful"). Was it that the coke is now laced with fentanyl? Highly likely, in retrospect.
The takeaway- your kid is already in the group that didn't quit right after college. If they stay in the group that doesn't quit within the next couple of years, they're going to end up in the group where over half of them are dead by 40. Or, if the coke is laced with fentanyl, they'll be dead much quicker.
Feel free to share this with them word for word. It is all 100% true. I, too, thought that doing coke at parties was no big deal at 19, and I completely agree that tons of people did it and everyone seemed fine, and it was normalized. Make them fully aware that a large portion of the people that did it in college , like them, have already stopped since they have moved on to other environments (law school, med school, other fields of work, other cities, etc). They're in the shrinking group that still uses it, and it will keep shrinking, and eventually it will start shrinking not because they're gradually growing out of the habit but because they're dying. The people I know who died were all happy, smart, successful, popular, attractive. It didn't protect them.