A couple of times I have been talking with people an they suddenly become deleted. I'm never sure about why they would have been deleted and it's frustrating that I don't know why since we were hitting it off.
Perhaps a small notification in the chat screen displaying a vague reason for account deletion would be good eg 'Account has been manually deleted by user.' or 'Account has been removed due to violation of forum rules.' This at least is less likely to leave people feeling like they did something wrong and the person abandoned them.