Don't do that, its not secure.

That amounts to Cargo Cult Advice.

How do you know he's not doing it on a private network not connected to the internet (for example?)

Maybe you want to say, "your password will go over the wire in plain text if you use telnet." Or something like that. But saying "don't do that" is generally unhelpful and completely antithetical to the spirit of hacking (in the original sense of the word.)

So, err, don't say that.