Hi, I had gone through your question and understand your concerns.
In my opinion if shunt does not work and your bf is well beside the headache it means that the
Cerebrospinal fluid somehow has found a way to get absorbed.
Headache has many causes and if there is no
hydrocephalus (which probably your doctor has seen in the CT) you should not worry.
As for marijuana there are several reports on it that it is no harmful than tabacoo, while a few others stating that it causes some
neurotoxicity which can be releated to some degree of the headache though in my opinion needs further studis to ascertain it.
Still, I would definitly exclude any link of marijuana with hydrocephalus.
Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions.
Wishing your friend good health.