Brief Answer:
Difficult

Detailed Answer:
I read your question carefully and I understand your concern about relatives/friends.

Rett syndrome is a rare genetic disorder affecting mainly females, leading to marked problems in the child's development and movement coordination. As part of its clinical manifestations may also be seizures. Unfortunately there is no known medication which can affect the course of the disease, one has to be realistic about that. Medications when used are employed only to alleviate the symptoms. In the case of Cannabidiol (CBD) it is used only with the purpose of reducing seizures. It is not an approved medication, still under study and used only when traditional antiseizure drugs have bee tried and failed. So unless the child has uncontrollable epileptic seizures CBD is not necessary.

As for finding it that is a problematic issue. Because it is a derivate of cannabis (marijuana) it is associated often with abuse. It is wrongly done so, because it is only one of the many components of cannabis and doesn't have the psychoactive properties related with addiction to recreational marijuana. It is changing as in recent years there is a lot of research about its use in medicine, however in many countries it is still not legally allowed. From what I am researching it seems it is still not regulated in Argentina. For that reason I am afraid that while you can purchase it in the US it is not possible to ship it. The package could be stopped by the customs and there may be legal trouble. The same may apply if you want to travel there to take it yourself, it might work just fine if you are not inspected, but if yes then you might have legal trouble for trying to introduce illegaly such a product.
There are debates about introducing it in Argentina, but still not achieved. The only exception seems to be the southern province of Chubut where local legislation has been passed to permit it, but that was done only last fall and not sure that it has already been introduced in the pharmacies there. If it is possible for them to travel in neighboring Uruguay or Chile it is available there, but still should be difficult as a prescription from a local doctor should be required. Perhaps they are aware themselves about these options as they must have done their research.

I am sorry if it was not the answer you were hoping for, but it is a tricky affair.

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