Don't think I used my email for anything important doing my time at FB. If it gets out of hand he could just make a request to have a new primary email made and use the above one for "spam"
But at least for email/calendar backend its exchange
The internal replacement clients for calendar and other things are killer...have yet to find replacements
For the most part though they use Facebook internally for messaging and regular communication (technically now Worplace but before it was just Facebook)
Email is really just for external folks