Perhaps a little logic will help open some doors. Ask for her reaction to this comment from a Catholic: "While I was attending a Protestant service the other day I noticed a woman praying to the Bible!" What would she say about that characterization? Would she say the Catholic misconstrued what was going on, that the woman was not praying to the Bible which she held in her hands with her eyes closed and the Bible pressed to her heart as she prayed, but to the God who inspired the Bible?
No one would jump to the unwarranted conclusion that this woman is treating her Bible as an idol. So why would she jump to the unwarranted conclusion that the woman she saw treated the statue of Mary as an idol? The same goes for having a picture of a deceased relative on the mantelpiece of her home. Would any reasonable person accuse her of ancestor worship? Of course not, so encourage her to use the same logic when she watches Catholics pray.
You might want to read Patrick Madrid’s book, Search and Rescue: How to Bring Your Family and Friends Into, or Back Into, the Catholic Church.