From the sounds of it Ryouta already knows, he had no surprise at all that the master had gone and his wife was there, I assume he plays along because he knows it's important to her for some reason. Either that or it was a plot hole.
"collapsed from exhaustion" does technically work, though... she exhausted all of the atoms in her torso & it's pretty hard to keep standing without a spine or any muscles connecting to your lower half
it's pretty much the same way a star collapses from exhaustion... by ceasing to exist