The issue of drugs - legal or otherwise, will always be one that creates allot of comment and differing opinions.
The only real way for these arguments to be solved is with extensive scientific research on the effects of drugs, legal and otherwise, over both the long and short term, with differing levels of usage and across a good mix of population.
Without this research then it is down to the individual to form their own opinion. Now we are back to square one. Without conclusive research you are forming your opinions off the back of somebody else’s, which means that whatever your views, you could be wrong.
I somehow doubt that this sort of research will happen, so we are back to trying to be sensible.
With the weed, if your friend is no longer smoking and hasn't done for a while. Then he will pass his drugs test (should he be tested) and because he has left his drug dabbling youth behind he will not be worrying about getting tested in the future because he doesn’t do illegal drugs anymore. So there is no ongoing problem for anybody to be worried about.
The mushrooms I know little about other than they are way more psychoactive than weed and there is the possibility of flashbacks. As Spartacan says try doing some research on the net.
To get you started click on this:
http://area51.upsu.plym.ac.uk/~harl/mushies.html
Cheers
Snoop