Nail down a deployment process. It doesn't have to be the sleekest CD pipeline you've ever seen, but it has to be few enough manual steps that I can do it consistently without breaking things.
Deploy to QA and do user acceptance testing of the current codebase, by which I mean I want someone who is in a position to know who can verify that nobody has been sneaking changes into prod without putting them into version control. (I have been burned by this every single time.)
Start making tiny changes.
a. The first round, put in some comments. Deploy.
b. The second round, gently fix some indentation or formatting. Deploy.
c. The third round, rename some things to make them easier to read. Deploy.
d. The fourth round, start breaking up giant methods into smaller methods. Deploy.
e. Continue a-d until you have some methods small enough and pure enough to unit test. Deploy.
Now you have living code, and, hopefully, with each deployment, it goes a little smoother and quicker and becomes a little more automatic and a little less scary. Eventually you can build a pipeline with what you've learned.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
It's a process.
a. The first round, put in some comments. Deploy.
b. The second round, gently fix some indentation or formatting. Deploy.
c. The third round, rename some things to make them easier to read. Deploy.
d. The fourth round, start breaking up giant methods into smaller methods. Deploy.
e. Continue a-d until you have some methods small enough and pure enough to unit test. Deploy.
Now you have living code, and, hopefully, with each deployment, it goes a little smoother and quicker and becomes a little more automatic and a little less scary. Eventually you can build a pipeline with what you've learned.