If they try to activate an OEM (not retail) windows 7 install and use that key, there's a 50-50 chance they will be halted with a message that they have to call a toll-free number to verify that the OEM version they are trying to re-use is not installed on another machine. If answered yes, at that point there's another 50-50 chance that they will be granted a new license invalidating the previous license... the one you are using.
It all depends on how vigilant Microsoft is when allowing an OEM license to be re-used on another machine. I have had no problem doing the above except for one time. That time I had to take one more step, and talk to a rep and explain why I was re-using an OEM license on another machine.
You should never...