Clarification: "Freon" is a registered trade name of DuPont for a number of halocarbon refrigerant products. It's more accurate, and definitely critical when recharging an AC system, to use the correct industry identification, like R-134a as used in modern cars, or R-12 as used in earlier cars.
As for leakage, a car AC system undergoes much greater shock & stress than a static residential AC installation, or in refrigeration units like freezers and refrigerators. A car AC must contend with:
- very high underhood temperatures
- engine vibrations and road jolts
- multiple disconnect joints in the lines, to enable car services
- a rotational seal at the compressor input shaft to deliver engine power
Static residential and refrigeration systems are largely sealed permanently, by weld or solder, with fewer disconnect sites prone to seepage. Therefore some slight leakage over the life of a car AC system can be expected.
Hence my first question to Sporty: how old is the car? Not only to know what refrigerant to add, but whether he needs a leak check with a detector, indicated if this is a newer car (4 years or less). Anything over 4 years then a partial recharge wouldn't surprise me (he says it still cools). If it loses cooling again soon after the recharge, then yeah, he's got a leak.