Tell her not to worry. The pieces of plastic are probably from bags used to bring waste to be shredded, or trash mixed with the material for composting. The plastic is not going to cause any toxic problems with her plants. Some of the plastic is probably photo-degradable and will disappear as the UV hits it; other plastic may be bio-degradable and will slowly decompose.
Renais

Plastic is perhaps the nastiest contaminate to see in your compost but it really has a chemical effect that I'd call unmeasurable. The stuff does decay but it's almost entirely hydrocarbons and they're very inert as far as what a plant can take up via its roots. This is opinion and not in any way is it scientific!

You could also, to help assuage any panic of hers, suggest that what she has growing is still far far kinder to her health than anything she could find at the grocery store, chemical-wise. Far better.

The above two comments indicate an acceptance of pollution that should not exist today, some 40 years after the first Earth Day, but it does exist.
Since there have been no real studies about the affects of plastic in the garden we really do not know what might happen.
There are those that believe plastics are inert and might be useful for loosening compacted soils, but we do know that some hard plastics release BPA in liquids. The Food And Drug Administration sets limits on how much plastics touching our food can release under the assumption that these are harmful to humans.
While plastics should not be in the soil of an organic grower what might be there is probably in small enough quantities to be of little harm. Some years ago some heavy metals were of great concern until we learned that plants do not uptake them, although if in the soil they may attach to the skins of root crops where they would be a concern.

Kimmsr,

The Bisphenol A was the first thing that came to my mind. Its really stipping my wheels. Thanks everyone for the replys!

Spinning*