A/C recharge?

Buying a car from a friend. His A/C used to work but the car sat for about 6 months not really being used and now the A/C doesn’t cool much at all. Can I just try to recharge it with the little bottle of r-134a? I know the correct way would be to vacuum it first and make sure there’s no leaks, but the car is for my daughter and we’re trying to save money. I didn’t know if there was any downside to trying this way first. I was hoping, worst case, maybe it would just not be as efficient but still cool the inside to a decent temperature. Thanks for any info.