The way psilocybin, the active ingredient in mushrooms affects the brain is similar to that of LSD, affecting neurotransmitters. This can lead to a number of psychological manifestations. Depression is more likely if the dose is large and the trip is deep. Contributing factors can be a tendency towards depression, or a bad mind set going into it. Perhaps your friend was thinking he was just going to get high, and wasn't prepared to undergo an intense inward experience. The setting is also important since some places have good vibes (like nature), and others can elicite negative responses (like cities). Perhaps most important is having someone supportive, who isn't doing mushrooms, around to monitor the situation and help out if necessary.

I'm surprised that people are doing these drugs (still) without realizing how powerful they are. LSD, psilocybin (mushrooms), mescaline (peyote) are all heavy hallucinogens whose dose is very hard to gauge. Each of these drugs has the potential of giving the user one of the most intense experiences of his/her life. They are not to be taken lightly, they are not for kicks or to get high. Many people take these psychedelics for the first time expecting a party. It may start that way, but often it turns into Scream, or some other horror movie gets shown at their own private cinema of the ego.

If you are prepared for this, knowing what might happen, and have the proper set and setting then the chances of a "good trip" are maximized. What is a good trip? I suppose it would be the one where the layers of the ego are stripped away, and the now humbled tripper sees himself as an infinitesimal speck in something so wonderful, that being just a speck, something alive and aware, is far greater than anything he can imagine.

What a long strange trip,

The Old Hippy