It’s not sane for him to complain about others but it will depend on how much he does it and what he does with it all after that.
Up to a point it’s normal, never sane, but something understanding that we vent about what we are living and stress us up.

He is lucky to have you. It’s clear he confides you to tell you about how he’s feeling. And only you who knows his background, his current circumstances and his character, can know how to proceed.

I’m not being of much help, sorry, but I’m like that, I need to know all details to know how I would respond. For example how much he feels effected by this ( because most of the times, the way people behave towards ourselves, it’s a matter of them and it’s not so much because of us. What it’s up to us it’s the way we take it) his background ( this is very important), you know.