Second tranche never happened. It's been more than a month now with nothing but excuses.
Is there a standard way to revoke a SAFE that is signed but never paid?
Odds are there is nothing that needs revoking. That being said, I have no idea what your contract says, so it is always wise to ask a lawyer, not us folk on the internet who aren't.
But again, it depends on what the contract actually says, so... they should still ask a lawyer.
Keep in mind that contracts, in general, aren't enforceable on their own to begin with. They are documentation of agreements that could be used to help decide conflicts in civil court, to get a court order that is enforceable. And they therefore often end up being leverage so conflicts don't go that far. So the only way OP has to fret over this is if they expect the investor may some day sue them for the investment rights, even after they stopped giving them investments.
When you have a priced round, either their SAFE is valid and they convert, or it's not and they don't, and they sue you if they don't agree with your interpretation of the contract...
But until that day, what're you hoping to accomplish now?