R134a is not a retofit replacent for R12 because of the oil in the comp.R49 is a retrofit gas.You have found the unit short of gas.There must be a leak.You must repair the leak before putting any gas in.Unless it is something loose then you will have to reclaim the gas to repair the leak,you might as well vac then recharge(do the job properly).By just topping up to save him money looses you money.Not your problem.He will have to buy new now or you do the proper job now.If you want to do him a favour tell him you will try to locate the leak for free.If you can't find it he will have to scrap it.If you find it you will be more confident to charge him for a proper retofit and he won't have to buy a new one.