This made no sense. Even though Xia Fei was not in the Assassins Sect, he knew Breath Control and Crafty Thrust, two abilities unique to them. Especially Crafty Thrust, only the top disciples who had trained hard years on end would have the opportunity to learn that technique. There was no reason for Xia Fei to possess it, too.
Xia Fei had a deep understanding of Crafty Thrust and Breath Control, and he was quite well comfortable in executing both, obviously a result of years of training. The Assassins Sect had always been strictly controlled; nobody who entered could leave, and they must stay in the discipleship until the end of their lives.
This point more or less removed the possibility that Xia Fei's abilities had been acquired from someone in the discipleship; thus, it could only be someone outside of the discipleship who taught Breath Control and Crafty Thrust to Xia Fei.