Im sure he will pass on some info but over time his ability to furnish Chelsea with current knowledge will diminish. Should he be seen to doing more than this, ie something which could be construed as work, he could be in a spot of bother.
Regardless of salaries most businesses will be in this position when top talent leaves and it is reasonably common practice. Looking at it another way, it protects/mitigates the loss of the club knoweldge and info to a potential competitor. It can also serve as leverage if you want to negotitate...