If it becomes true then we have predicted the future and must add it to the list of powers.

"Then it's settled!" And he changes the hand to now be open towards Viserys for the expected and important handshake to cement their newfound friendship for the days to come. If he was to show Viserys the ways of this place, the ways of friendship must be included as well.

Plus, Gentarou is in high school. And just like any high school based situation, you always have that one friend who's a pain in the ass little shit that you deal with anyways. That's just how it works.