If you can swing it go ahead and buy the A/C window unit.....deduct the cost of it from your rent......and keep it for a "just in case" time that your A/C goes out again. Buying it with the sole intent of returning it after using it is wrong. I can't believe she suggested you do that!

Whatever window you decide to put it in she can know that it will stay there after you leave so she can use it as an "extra" for the next tenant. They'll have a "cool room" should the A/C ever go out again.