How long can the dude possibly tolerate to be in that company? He's hopelessly left behind already, with his sometimes accurate, sometimes inaccurate knowledge as his only edge. The situation ought to start gnawing at him and erode his psyche because it's only going to get worse. Furthermore, at some point the enemies would be too easy for his students, whereas they would still be tough opponents for him. That would start to create a rift also from the students' side, which, again, would be made only worse because they feel indebted to him. They wouldn't feel good leaving him behind (basically kicking him out), but they would also feel he's useless, deep down, and holding them back. It's the worst possible situation. It would be the best for both sides if they parted ways until Rex can fix his stats, if it's possible.
The sister would try to stick with him, though, and probably try to protect him as much possibly, which would probably damage his efforts to get stronger. Although since you can't predict everything in a living world, maybe she would also keep him alive.