Should I warn a professor about his bad internet security practice?
If this was a one-time thing and you don't know this person very well, I would avoid bringing it up.

Once you know this person fairly well or communicate with them on a semi-regular basis, then sure, in your next email to the person just put as a note at the end of the email that they might be cautious about including the entire webpage.

The concern is that it might be off-putting to someone if they don't know you well and perhaps this professor actually verified that no personal info was included.