Yeah emptying his entire inventory seems goofy and quite an asspull with what we know about his inventory UI, but I can't blame him too much for not seeing the incoming betrayal. People are always vulnerable after a win. He's surrounded by powerful allies that should know Qubi far better than him and even they didn't see it coming. I'd blame his allies far more for not catching on to Qubi, especially depressing that only Aira quickly assumed the culprit/issue. Easy to forget the MC only knows what he's told/experiences of the world, he's like a child, they really shouldn't be leaving him alone...like...ever.
They had been constantly bringing up how precious the MC is and then they just leave him alone in previous enemy territory just because Qubi was with him. Yeah, they fucked up far worse, disappointed in Sylphy especially. MC should have a team of body guards on him constantly if he's somewhere even slightly dangerous. I feel like the writer could have found a more reasonable/realistic way of splitting the MC up and making him vulnerable here, like drugging him.