Is it ok for guinea pigs to digest cardboard?

Yes! Definately! I know of guinea-pigs that eat it all the time! It is normal behaviour and is simply alike to us humans eating biscuits. Don't worry! Other weird things that guinea-pigs will eat include; Wood Their cage! Newspaper PS. Never attempt to feed guinea-pigs, (or any other creature for that matter), any substance that may be regarded as harmful.