Lenny made a mistake in his proof with the statement regarding the angle notation. The correct assertion should be that the angles ∠LMI and ∠LMJ are right angles, not ∠MLI and ∠MLJ.
Thus, the correct response is:
"The definition of a perpendicular bisector tells you that ∠LMI and ∠LMJ are right angles, not that ∠MLI and ∠MLJ are right angles."