In this case in particular, depends whether he broke their terms of service, which is by itself more arbitrary than the law :/ I don’t think the guy did, so maybe he can win in court :)

By the way, if you break the rules in your instance, you gettin’ banned, so Lemmy is the same. The cool thing about it is that the rules vary from instance to instance and you should know what they are before you federate with them or open an account there.