Your pet shop owner was wrong. Green poo means that your guinea pig has eaten some bad grass...or, in this case, cabbage. A food not fit for a guinea pig.
Being vegans, certain veggies are choice for piggys. Grass (not lawn clippings), dandelions,and clover. They will also like celery, cucumber, sweetcorn (maize), capsicum pepper, carrots and fruit. More convenient and cheaper is dried mix from the pet shop. A large bag will last for ages.
So... not meat!!! Give your piggy meat and your guinea pig will be indignant and likely move into a commune for vegan hippies, leaving you sad and lonely!!!