> probably against GitHub's ToS, but

Why would running code on a github action runner that's built to run code be against ToS?

If it was, I'm sure they'd ban the marketplace extensions that make it absolutely trivial to do this: https://github.com/marketplace/actions/debugging-with-ssh

could have just used https://github.com/mxschmitt/action-tmate

So this friend escaped from the ephemeral container VM into the build host which happened to have a private SSH on it that allowed it to connect to a bastion host to... go back to the build host and debug a failed build that should be self-contained inside the container VM which they already had access in the first place by the means of, you know, running a build on it? Interesting.