Happened to us 20 years ago. My late cousin’s wife came with her sons for the seder. She walks in and places a ‘Passover cake’ on the sideboard in the dining room. My wife took a quick look and saw that it was from a non-kosher bakery. She quickly thanked our guest, explained that tonight’s menu was already set and instructed the eldest daughter to place the cake in the garage where it would stay cool. Daughter picked up the cake in its box along with the cloth that was on the sideboard and walked to the garage. Somehow the cake managed to end up in the compost heap behind the garage.
After Yuntif, we sent a lovely note thanking cousin in law for the cake.
We couldn’t get upset with her. She was not brought up with any religious knowledge. Her husband passed away leaving two young sons and the annual sedarim at our home were their only connection to their Jewish heritage and our side of the family. Wife explained that we eat so much rich food for the holidays that we only serve fresh fruit for dessert. The following year she brought a bunch of bananas.