Mushrooms?
Mushrooms, particularly raw ones of certain variates (I can't remember them all but they are ones commonly eaten by people raw and cooked), can contain toxins that can affect people negatively. Thus I wouldn't risk it. As far as I know cavies aren't really designed to eat fungi like mushrooms.
That's kind of what I figured. He wanted to exactly why they couldn't have them. I told him that they weren't listed as safe and he asked why, again. I figured it was because they were a fungus but told him I'd ask you guys. The piggies enjoyed a nice treat of grass today. We're hoping to take them outside some this summer in a modified store cage that we use for travel.
I honestly can't think of a good reason tia. I agree completely with better safe than sorry, that they have the potential to be toxic, and no real nutrition. When you balance the risk with the lack of benefit that's the only real reason I can come up with. I think those are very GOOD reasons, and I'd never feed them, but I don't have the kind of answer I think he wants.
Pigjes, yes that's the toxin I was thinking of, thank you (I couldn't for the life of me remember what it was called). I know it's not great for people either, though that doesn't stop some people from eating raw mushrooms anyway.