Firstly we need to say that no concrete medical studies have been carried out to determine whether drinking cranberry juice can help with kidney stones or not. Contradictory advice has come from various medical establishments as to whether or not cranberry juice and kidney stones are linked.
Another thing to note is that there are different types of kidney stones that are formed from different substances and so this fact in itself may be one of the problems about whether or not cranberry juice helps with kidney stones as it may depend on the type of kidney stone and what it is made up from as to whether or not cranberry juice can help with reducing or preventing the kidney stones from forming.
One of the most common forms of kidney stones are those made up of calcium and oxalate and although cranberry juice does increase the concentration of two substances that help to prevent these types of kidney stones, it also increases the concentration of calcium and oxalate themselves thus negating any positive effect that may be seen.
There are other less common forms of kidney stones that are helped by drinking cranberry juice however as these kidney stones are far less common, unless you know exactly what type of kidney stones you have then it might be advisable to steer clear of cranberry juice.
Other juices like orange juice have been shown to have a better effect on kidney stones than cranberry juice as it does not increase the levels of calcium or oxalate so this may be a better bet.
But really, if you have kidney stones then the best thing is to consult a medical professional so that you can get proper medical advise on which is the best way to treat kidney stones.