3 days pass, and somehow I get the feeling he hasn't explained anything to his friend. I mean sure a full explanation would be needed to dispel her doubts, but the base reasoning behind his growth isn't that difficult to explain.
"One of my skills has allowed me to gain incredible ammounts of SP. That is how I've become capable of fighting like this. The catch is gaining levels would would actively hamper my growth. So this only works if I act without a party."
But logic seems to be anathema to most manga settings.