No. The social security info is used with the carrier not the phone, so you won't see that.
To perform the unlock, the Carrier would need to validate it is you requesting the unlock and that you qualify for it. That's why I suspect he needs the info.
Again, you are under no obligation to help them unlock it for use with another carrier.
Standard activation of the Phone can be done with a functional sim from the carrier, there is no need for social security information.
If you want you could contact the original Carrier (AT&T) and request the unlock yourself if you want to help them unlock it.
Other than that there's nothing else you are required to do.
I disagree with Phil0124's assertion "You didn't necessarily screw up".
The two paragraphs he included discussing possible motives both say the social security number is not required to do either. Yet the purchaser did ask for this information (last four).
I conclude yes, there was a screwup for this reason.
What screwup exactly? The only screw up would have been if Grants had given the buyer the social security digits.
Other than that, there is no screw up if no unlocking was advertised and no digits have been given.