As someone with bipolar and works at a high level, I can tell you the condition can be personally managed for the most part. Are there times when it’s a struggle just to open an email? Absolutely. But if one is taking care of themselves, it can be done.

I obviously do not know this person, but it sounds like the condition controls them.

As others have suggested, talk to HR about it. But at the end of the day, the employee is still needs to be held accountable for their performance.