You can log in with his nPub. You would have to log out of your own account first or try another (web) client.

Here is an example of what everybody can see of your own DMs when logging in with your nPub.

We can see who you sent messages to and when. Jeff in this example. We can just not the content which is encrypted with your secret key.

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Indeed. Wow. Thanks for helping to grok this better. I stand corrected.