Hi MagentaMage,
There may be a few different things to try:
Does the Foundy have the skills to actually cook on the fire?
They will require their Farming skills to be upgraded to ‘Cooking at Campfire’ on their Professions, when they have gained a point from experience at farming.
Once they gain that skill, then it should be just selecting your farming Foundy and right clicking on the Camp Fire and ordering what raw food you need cooked - as the above image shows.
If you still have problems, just reply and we will work through it 