In the wild, piggies probably eat whatever they can find!
But, your piggies have access to all the tasty tasty veggies you can get for them from the shops, so it's not really worth the risk..
Allthough from a guinea pigs perspective they sure do look tasty!
I know some plants can be dangerously poisonous, especially to smaller animals. Foxgloves for example! And alot of small berries actually have poisonous flesh as an attempt to protect the seeds from getting eaten.
I'm not a plant person, so I dont know which wild plants would be good and which would be fatal, so I'd never take a risk, no matter how tasty those leaves look!
Did you know Doc leaves can be used to soothe stinging nettle bumps if you're out walking and get stung?