Romaine lettuce is good for a guinea pig because it is relatively high in nutrients. However, you should also feed other fruits and veggies as well as a guinea pig pellets. For more information about caring for and feeding your pet guinea pig, check this site - http://www.2ndchance.info/guineapigcare.htm
A guinea pig can eat one large leaf or romaine lettuce every other day. They should also get a variety of other fruits and vegetables as well as guinea pig food. Be sure never to let your guinea pig eat iceberg lettuce since there is virtually no nutritional value for them.