This is the issue with letting employees use personal devices on your corporate infrastructure. The best thing to do is to put language in your employment contracts / acceptable use practices that clearly states if an employee decides to connect their personal devices to the corporate network, should they decide to leave the company it's standard procedure to remote wipe their device (regardless of what personal data may be on there). This way employees have a clear understanding of what will happen when they leave, and backing up their device becomes their responsibility.
To that same note though, if you wipe it who's to say they haven't done a backup of it already anyway (which they could then just restore to get to email and other info). Until you get that language into your company policy you're probably fine just deleting them from the BES.
Device: 8100c / Apps: 184.108.40.206 / Platform: 220.127.116.11