Welcome to DF!
On a first note, for questions like these, please provide more information on your friend's background. How old is he, what is his diagnosis, is risperidone the only drug
he takes, etc.?
Generally speaking, there is no reason to suppose that a patient who is on risperidone and who smokes cannabis
is going to die or suffer life-threatening side effects
, but that's about all the reassuring that one could do here. A scroogle search yields a report on a different forum from a user who reports severe anxiety attacks when smoking cannabis while on risperidone for example.
Besides that though, it is absolutely possible that cannabis use interacts badly with the medical condition itself for which your friend is taking risperidone. My advice here would be to definitely
lay off all drugs
until your friend is stable, i.e. if he is experiencing mood swings and the like, cannabis (or any other mind-altering substance) is only going to blur the clinical picture, making it more difficult or impossible to adequately assess and treat your friend. If he is stable, he should ask himself some serious questions to which only he can know the answer, notably: what does he expect from cannabis? If he decides to take the risk to find out how it works on him, tell him to start with little amounts and
to only do it in the presence of someone else who knows him, his condition, his medical regimen, etc. Antipsychotics and the indications for which they are prescribed are really not be fooled around with.
I hope this information helps!