It might be possible to do this, if no one else is running CS2 with the same activation key — Adobe allows you to de-authorize a computer if you’re no longer going to be using CS2 products on it. So if your friend still has it on a machine, you can have him de-authorize the machine it’s currently on, and then you may be able to activate it on your own machine. It’s a bit of a grey area, however — while you’re allowed to have Adobe products running on the same license key on two different computers, you’re technically only allowed to have one of them running AT A TIME, which means the license agreement is designed to allow you to use the same copy of the software at home or at work, but not both simultaneously. That means that if you’re not the only person running on that specific license key, you may both end up with invalid licenses if they both happen to be running at the same time.