It would be best if you use the ones you got for them I think. Yours would probably work initially on theirs but may cause problems at update time (program/iAVS).

I don’t know how the registration is validated, you can have multiple copies of the home version (provided they qualify for home/non-business use) and the same reg key without problem. However, using the same reg key and using a different ISP/IP address may (my thoughts only) cause a complication.