Your son can get sick being fed animal product if it's not normally in his diet.

I get where you're coming from (I've been a vegetarian for decades - just cross-contamination will mess me up), but OP mentions their child is only a year old - there's probably a whole host of foods that have yet to eat that have nothing to do with lifestyle and everything to do with the kid having only been introduced to solids six months ago. If a little meat made kiddo sick at this point, it would be rather alarming.

But I absolutely agree with your point that grandma literally told OP she was planning to do this. Honestly, I'm not sure why they're so surprised in the first place, they'd been given clear warnings well in advance.