A healthy cavy needs fibre. Hay contains 25-45% of crude fibres, while veggies basically none - carrots for eg. contains 1,2-2,4%. Their sugar content could be high, which is another bad point.

Since cavies are grazing animals, they do not need any veggies or fruits. They can live a full and healthy life without suffering any harm.
The risk is to feed veggies or fuits, not the lack of them. It is all about humans. We want them to be happy and they undeniably are into unhealthy snacks. But unfortunately they have no clue about their needs, they can consume poisonous plants without any difficulties. Thus, what they are into should not be relevant. Their well-being is entirely up to us, it is our responsibility to think through what's best for them. They won't eat anything that we do not give them to eat.

So, when you want to feed snacks, the healthiest option is to pick herbs or grasses that aren't suitable for constant feeding but occasionally are okay. Dandelions, legumes, oxeye daisies or raspberry leaves are exceptionally great snacks.

If you still insist on veggies or fruits, at least follow these guidelines to reduce the harms as much as possible:

* chemical residues could be lethal, so never give them veggies or fruits of unknown origin

* choose seasonal, local species, and the organic ones

* do not feed plants with high sugar content – for this reason avoid almost all fruits

* plants that are known to cause bloating must be avoided, it could be a life-threatining condition for cavies

* green bean, potato, rhebarb and beans are poisonous, never feed them

* avoid the different kind of lettuces (romaine as well), even organic ones could contain toxins, they can concentrate  them very well

* do not feed more than a small slice per day

You can always hear accounts that certain cavies lived 10+ years by consuming nothing but lettuce and sunflower seeds. Or with less exaggeration, you can read about animals that live on unhealthy diet for years and without any medical issues. These cases can be true. Not every smoker get cancer either. What you should consider is the possible ratio.  And although some cavies are lucky, most of them wouldn't live a long and healthy life if their diet does not follow the scientific guidelines.

Cavies are generally neophobic, so the large variety of snacks are unnecessary and it can increases potential risks. The few veggies or fruits that you give them should be 100% safe.
The ones with the lowest risks are:

* organic, local apple or pear from farmers – not the sweetest kind

* local butternut squash

I know, the list is very short, but thanks to the climate of Ireland almost all veggies are imported or grown intensively in greenhouses. 

The safest way is to keep them away from veggies or fruits. These cavies in Ireland are extremely lucky with the large quantity of available grass. Most cavy owners in the continent would kill for this opportunity, while many locals tend to ignore this grand gift. Do not be one of them.