My point isn't whether he can be a good Catholic. It's whether he's done something that would prevent him from being friends with Ratzinger.
I'm betting there are some things that are such serious crimes that the Pope wouldn't really relish the presence of the man.....avoid the appearance of evil kind of thing.
I don't think Kung's violation is of that nature.
My impression is that Mr. Kung doesn't believe significant portions of the Catholic Faith. The only reason for a person to make an issue over the infallibility of the Pope, is that he disagrees with doctrines that are infallibly defined :-).
However, that simply makes him a Protestant, or, if he disagrees sufficiently, a non-Christian. I have plenty of friends (and FRiends) and cordial acquaintances (not to mention parents and siblings) who are Protestants or non-Christian, and I don't see why the Pope shouldn't.