"Radicalizing" someone isn't a crime and shouldn't be.

Actively getting a random person to join a terrorist organization and then fly overseas to fight for it is a crime; it's the crime of "Providing material support to terrorists" (18 USC 2339A), which (sensibly) includes "personnel" in its definition of "material support".

As usual in Message Board Law, we're here being asked to fall into the trap of considering each of this person's actions independently, ignoring the greater (and in this case crystal clear) pattern of their intent and the outcome of their plan. "Putting two people in contact is not a crime". Sheesh.