I did some (simple) research on this recently and was unable to find an absolute yes or no for the question "are tomato leaves poisonous?" I don't think your piggies will suffer any long-term (or even short-term) effects, but if there are so many other safe veggies available that it's probably not worth the risk to continue offering them, IMHO.
Aren't they part of the Nightshade family? I have always been told they are toxic, and even touching the stalks and leaves causes me to get a rash. Honestly, you may consider contacting your vet and talking about potential organ damage that could occur.
Looks like the latest research says that they're not toxic to people, but I can't find where any studies have been done in guinea pigs. Given that pigs are so much smaller than humans and would require much less of a toxic substance to make them sick, and have such picky digestive systems anyway, it would probably be better to skip them.