Idolatry is worshiping a statue as a god. His mother-in-law is certainly not doing that. I don’t know how she explained it to him (she may have expressed herself badly or he may not have been listening), but based on the description of her explanation you have given, what she is doing at most would be superstitious, not idolatrous—unless she worships the statue as some kind of flower goddess (not likely!).
Bottom line: If she thinks the statue is a god, it’s idolatry; if she thinks it is not a god but has intrinsic power, it’s superstition; if she thinks the statue has no intrinsic power and by placing the statue she is asking Mary to pray that God will bless her garden, it’s prayer.