Ask him not to do that - he may have though he was doing you a favor, and most people cut their grass shorter than is good for it. Nicely explain you keep your lawn in a certain condition and just prefer to do it yourself. Of course, don't ever expect him to help out at your place in the future, or take care of things for you when away on vacation.
If on friendly terms, explain your lawn cannot take short mowing in the summer - otherwise if not on great terms don't point a finger at him. A single short mowing won't kill it - you might have to increase watering for a couple of weeks, but should come back quickly unless an exotic or long-stem grass.
