Families are sometimes really bad things to be a part of.
Take this Petty Revenge post where the OP is getting kicked out of his house so the father can continue to lie to his foreign girlfriend about still sleeping with his wife/ex when the foreign girlfriend comes for a visit.
I think that there could be many worse things done than a spilled can of tuna juice. I do like the pregnancy test idea. My suggestion would be to hide photos of the father and his ex around the house for the foreign girlfriend to find, as well as hiding things that will definitely show the father is lying about his relationship with his ex.