You might also try to play to self-awareness: "Have you noticed that when the subject of x comes up, you have to go through the same ideas, in the same order, said in the same way, every time?"

[wait for answer]

If either of them notices, you could then describe the effect this process has on others: people stop communicating, because they know what's coming, and they've already heard it.

You might want to check some Aspie self-help sites or books, too.