I would just find another job, let them crash and burn on their own
Best option is to just leave. OP can still talk to the employees that he is friendly with - maybe that will help them take the situation seriously
If there is litterally not a single line of code in 10 years and nobody does anything all day but drink kombucha and play foosball then they already know...
Equity is a gamble and should be treated as such. Most jobs don't contribute any value in the grand scheme of things beyond shareholder value, so that's not a loss either. If anything it's a learning opportunity to not trust startup founders so blindly.
But if you want to warn your coworkers out of comradery, do so in a way that isn't simply telling them "I think our boss is defrauding investors".