We had the air conditioner serviced last week they fixed a leak in the coil and filled it with freon but yesterday it started blowing warm air again

Firstly does the outdoor unit run? If so have you noticed any excess water around the furnace? I ask this because for a period as the refrigerant leaks from the unit the indoor coil will freeze. Usually people notice water does not drain properly as the ice diverts it away from the drain. Regardless it is warranty! Any company worth its salt offers at least a 90 day on their labor. Its surprising that joe cooling chose to repair a coil... the leak must have been in a very serviceable location as it is a general rule to completely replace the indoor coil due to low cost. It sounds like you called the cheapest guy in town. It is very easy to verify you have repaired a leak properly following a repair. When he comes back ask him what value his micron gauge blank off was. When he looks at you like you have 3 heads ask him if he owns a micron gauge he obviously did not use one. A micron gauge measures quality of vacuum. Following a repair prior to charging a unit a vacuum must be pulled on the system to remove contaminates. For a standard residential system it is common for a vacuum pump to run for 2 to 4 hours to achieve this properly. Should a leak be present a micron gauge will bounce through the roof and tells hvac guy he has to re-check. Most joe cooling contractors do not do this as it is costly to the customer and joe cooling will complete the repair for much less and plays the odds on warranty repairs. Next time hire a quality union contractor, pay the extra bux and know that your compressor will not burn out in 6 months due to contaminates. I say union because they have no excuse. The union provides and usually forces quality training and certification training. Joe cooling could be your gardener with a set of torches.