Gee skippy, my own mother lit candles to saints. It's still idol worship. Mexico is known for mixing Catholicism with pagan worship. I'm not calling your FIL a drug dealer. I'm suggesting those that make offering to statues are pagans.
Muerte was an extreme example of idol worship. A better place is Isaiah 40. Why do you offer gifts to statues? Do these statues eat or drink? Do flowers please them? Do the eyes of these statues see light so that you must light candles for them?
What power do they have? They cannot move on their own so you must carry them from place to place. Yet you sacrifice offerings to them in and attempt to entice them to act on your behalf.
The curtain in the temple was torn in half. You can go directly to the Lord. Why does this bother you?
Do you prefer small gods that you can carry around?
But they are not. They are merely a cultural respect for saints who are seen as a positive force in one's life. And the Lady of Guadalupe is venerated across Latin America.
I see no downside and, although I hesitate to speak for God, I suspect He accepts the offerings in the spirit they are meant.
God is not a prig.