> How can you be honest with someone like this without him being able to turn it into an attack?

The solution is gray manning. Don't react to peoples emotional swings, controversial statements, and throw everything they say to the wind but the nuts and bolts. You don't know this person, you don't need to like them, you just need to exist with them for a bit.

Some people from the corporate world I like and become friends; everyone else gets a git clone --depth 1 when I'm around them.

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There are a lot of plausible explanations for his behavior. In some he's a bonafide victim, in others he could have a personality disorder. To you, as a transient existence in his reality and vice versa, none of those matter.