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I read several definitions of rennet, and they all say that it comes from the stomach lining of suckling calves, so, presumably, they do not have grains in their systems yet. Still, I appreciate that folks post their thoughts and concerns - it allows others to investigate something they might not have otherwise thought of. She could, after all, have been right! And she may yet be right, as apparently there is a confusing oxymoronic term "vegetable rennet" which is a plant derived product for which I cannot find the ingredient listing.
Hilary

 RENNET [rennet] substance containing rennin, an enzyme having the property of clotting, or curdling, milk. It is used in the making of cheese and junket. Rennet is obtained from the stomachs of young mammals living on milk, especially from the inner lining of the fourth, or true, stomach (abomasum) of milk-fed calves. The preparation of rennet was formerly a part of the domestic function of making cheese; the inner membrane was kept in salt, dried, and, when rennet was needed, soaked in water. Now extract of rennet is made and sold commercially. It is usually prepared by soaking the tissues in warm, slightly salted water and straining and preserving the resulting liquid. Heat interferes with the action of rennet. 

Author not available, RENNET., The Columbia Encyclopedia, Sixth Edition 2005 http://www.encyclopedia.com/html/r1/rennet.asp

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