Do unto other as you'd have done to yourself. As far as what they do unto to you? Well that's another subject ...
If you're going to make a move on insider-information, even though it's not illegal, it's definitely not honorable. However, if it is public knowledge that he's getting out of the business then it may not be as sleazy. But those accounts may be the most valuable thing he is selling, so if you do something to reduce that value, it wouldn't be too cool.
I'm all for hustling up work, but if you start stepping on other people's turf, you're setting yourself up for a fall. Aside from a LCO dropping the ball, just how loyal do you think these newly-hawked customers would be to you? I try to avoid price-shopper customers.