well, it is quite possible that they would have had serious medical ramificatioons by soley abusing the fentayl patch...
however, cannabis does indeed have medical effects & can alter a person's reaction to other drugs...
i have heard that pot can slow down the liver's turnover rate for a number of drugs (including "downers" and opiates) - so it is not impossible that the pot/opiod combo was just a wee bit too much
(& the kid that lived was prolly fubared due to lack of oxygen to the brain for too dam' long (anoxia/hypoxia) & turning all sorta bluish, eh?)