Papain is found in pineapple, and many people will given pineapple chunks or juice to help with this problem.
You can also try a broad spectrum multivitamin tablet as occasionally this behaviour can be triggered by lack of other micronutrients.
However we often find this is a purely behavioural issue, and prompt removal of the faeces is recommended. You can also try some over the counter products that make the poo taste bad (apparently it doesn't normally!) such as Deter tablets.
Very very occasionally the abnormal appetite (we call it pica) can be an indicator of underlying diseases such as liver issues. If you wanted to be really thorough then you could ask your vet to screen for these diseases. However, the chances are Holly is just conforming to the typical labrador stereotype!
I hope this information helps.