Do you know someone who works at NSA? I knew someone (a rank-and-file SIGINT engineer), and he swore that people in his office are trained to discard any domestic data they encounter. Was he lying? Maybe. Did the policies change? Maybe.
The likely scenario is that information distributed throughout the agency suggests that they do not spy on US citizens. Basic training is done for all employees where they are taught not to spy on US citizens.
However, NSA also conducts clandestine operations. Employees selected for those operations are probably recommended based on their willingness and trustworthiness when it comes to spying on US citizens and effectively breaking the law.
Alternatively, as has been suggested recently in numerous articles, FBI probably works hand-in-hand with NSA to "legally" allocate information required for datamining. To the employees of the clandestine operations, the information obtained is technically "legal", so perhaps that serves to settle the nerves of the employees involved.
I think the most likely answer is that your friend was not lying, but that the NSA is a massive agency with numerous different divisions and operations that do not all follow the same set of rules or have the same objectives.
