Inherited a codebase with no documentation and no original developer

What we told them

Do not attempt to document it top-down; you will run out of patience before it is useful. Instead write tests around the behaviour you actually depend on, one workflow at a time, starting with whatever costs the most if it breaks. Tests are documentation that cannot go stale, and they convert fear into a safety net incrementally. Within a month the parts you touch daily are safe to change and the rest can stay mysterious without hurting anyone.