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I've been in contact with this partner for a few years now, getting coffee/lunch now and again and obtaining career advice. I just noticed that he was ranked on Chambers for the first time this year. I will be seeing him again soon, and I'm wondering if it's appropriate for me to bring it up / congratulate him on. Getting ranked on Chambers is a pretty big deal right? That partners would be proud of?

Yes, getting ranked by Chambers is impressive. Especially since you've been in contact with him for a few years (and not just made his acquaintance), by all means feel free to mention it.