Been lots of genocide that the UN didn't respond to- Cambodia comes to mind first....Probably one of the worst in world history- but neither Ford nor Carter responded- nor any UN countries because we'd just got our butts handed to us in Vietnam and intervention could have easily ended in a full fledged war with China who were supporters of the Communist Khmer Rouge....
In this case in Nigeria, I would have no problem sending police aid/drones, equipment if Nigeria requests it.... Because these folks are nothing better than a criminal cartel- wanting to sell these girls as slaves... And the US and many foreign countries have law enforcement mutual aid agreements to assist each other upon request in such operations... Much as we are currently aiding policing pirates off Somalia, Sumatra, South and Central America, Mexico , etc. ...I believe the slave trade is also banned under international law that we have signed onto...