If he uses a KYC exchange, even for small amounts, could authorities link his wallet to him?
Yes, as centralized exchanges gladly send the documents in order to have license to operating in each country.
Could banks or tax offices freeze his account or ask for explanations if more money starts showing up?
That depends on how his bank looks at crypto.
Should he convert ETH to BTC first? If yes, how to do it without linking it to himself or using KYC exchanges?
Generally its easier to sell BTc via p2p, and to do that exchange, probably the best is
https://exch.net/ (which is also advertised here as well).
How does an in-person P2P trade actually work? Any dos & don’ts to stay safe and not get scammed?
That's the tricky part. Generally you want to meet in a public place and not in small small alley in the middle of the night and its good to be able to verify whether cash that you are getting is real and not counterfeit.
By the way, here in Croatia we can sell up to
$1000 euro worth of crypto on the daily basis without KYC so if he is not too far away from us he can simply travel here, stay few days, sell his crypto kycless and go back to Germany with cash in hand.
