However, trying to duplicate it inside the Stack Exchange network gave an error "The answer must exist on Ask Ubuntu" so I just linked to Super User without copying the answer.
Good enough? Or copy-paste and Source?
If it cannot be marked as duplicate, it means that the system as of now implicitly does allow duplicated question to exist over the network, so I think it should be answered.
Plus in this case the question is tagged
ubuntu, but in general an answer given to a question in a particular network might not fit well if given to the same question in a different network, i.e. the same question might have slightly different answers over different networks.
So the best thing to do would be:
If he's not or he's not signed up on AskUbuntu then, as for regular answers, I think the "The link might go down" and "Posting links in answers without an explanation does not make up a good answer" criteria should apply, so in that case you should: