I think you should contact your company's HR department and talk this over with them. You may find out that they have a corporate policy of discouraging peer references; some companies even discourage manager references, preferring to retain for themselves all communications about employees.
(For the sake of your ex-colleagues privacy, you may want to hold back info like the name of the new employer and exactly how long the colleague may have been planning to leave.)
The point is, given that you still work there, the employer may have some responsibility for your comments. And as others have said, negative reviews can be very risky.
Having said that, given that the incident was after your conversation with the new employer, and presuming that your comments were reasonable and fair at the time they were given, and especially assuming that you had no reason to expect this person to act like this, I don't think it's worth reaching out. The experience you have with this person over the course of your history with them is more likely to predict their performance at the new job than this incident is. And providing a reference is a point-in-time data transfer, it is not an ongoing commitment to monitor a person on anyone's behalf.
