Fruit should not form a regular part of their diet (in fact it doesn't need to be in their diet at all). Any citrus fruit is too acidic and can lead to a bacterial infection in their lips. Best to avoid citrus. It is too high in sugar and isnt good for them. One tiny piece of fruit as a treat no more than once per week is fine.
Safe daily veg include lettuce, a green bean, cucumber, bell pepper and coriander/cilantro. These veggies give a nice mix of nutrients including the essential vitamin c.
The majority of their diet needs to be hay and if you have access to dog/fox pee free grass then picking some for them to eat is good. Its their natural diet. Always introduce grass slowly to not cause digestive problems.
Long Term Balanced General And Special Needs Guinea Pig Diets