Percy could have followed orders and detained Nadine, preventing her from warning the innocents in Africa of what was coming, but she decided to go with her gut.
Unfortunately, as Shalita Grant said, Percy made an error in judgment, and that error will land her in prison.
Nadine Bancroft was pretty clearly a shady individual, what with all the funneling of stolen weapons out of the country and all.
Still, she earned some sympathy from Percy when she claimed that she was sending the weapons to African villagers who have been terrorized by Boko Haram, and that sympathy was enough that Percy was convinced to let her escape during a gunfight despite strict instructions to bring her in.