I'm pretty embarrassed to read these responses... Lots of opinions but very little law.
1. Your average unit should last around 15 years. I know you are close to that, but have you gotten second opinions from alternate HVAC companies?
2. If this unit is in fact toast, you are under legal obligation to replace it in a timely manner. Window units are an acceptable solution for a few days while you get a tech scheduled, but it is not a long term solution. These slumlord esk answers of "screw the tenant and put a window unit in," are ridiculous. The tenant rented the home with functioning AC, and you as the landlord (unless your lease relinquishes you of your responsibility) are required to maintain it.
3. I'm a property manager of over 325 units and and investor myself with nearly 20 rentals, so I'm not someone who hasn't been here before. It sucks, will wipe out your profit for the year, and isn't fun to deal with, but it needs to be done. Its the only ethical way of managing a rental property and you will go another 15 years without having to think about it again!
Best of luck to you!