Call a garage door installation company, and have them look at the dents. Ask if the dents can be hammered back without too much cost. OR, you could do it yourself.Maybe take a flat board, say a 1x6 about 2 foot long, and use a rubber mallet to hammer on the board to hammer the dents back the other way again. It might not be back to "perfect" again, but the door probably does not need to be replaced, either. Take pics of before and after, and charge the tenant $5 per dent.