Rabbits do not have the ability to cough up hairballs. Like rats, they have no "upchuck" reflexes. It is important that you try feeding some fresh pineapple or dried papaya to your rabbit, just to be certain that there are no wool/fur blockages. These will help to dissolve any fur/wool in the digestive track before the blockage becomes fatal. Yes, rabbits can die from hairballs. To prevent future hairballs, make sure your rabbits have plenty of access to hay -- roughage/fibre will also help to push blockages out.
Also, you should be careful when feeding your rabbit fresh grass. Any food with high water content should be fed with caution, as it can upset their digestive systems. Keep an eye on their poos -- if they should become soft or runny, stop feeding everything but hay and a bit of pellets for a while. If the problem doesn't stop shortly (like within a day), you should take them to a vet -- diarrhea is another little problem that proves fatal to rabbits as moisture control is absolutely critical in their diets.
This doesn't mean you cannot let them eat fresh grass, just that you need to be careful they don't eat too much before their systems are used to it.
Glad to hear that Coco enjoyed her time out today.