She could have a bit of a point, depending on the design of the bird bath.
If it's like the one we had when I was a child, fairly large and flat bottomed, it could trap water underneath it. A solution to that possible issue would be to elevate the birdbath in one way or another, thus allowing airflow to dry the wood out.
That said, I'd guess the wood is preassure treated, so really shouldn't be an issue.
/Mats
_________________
Interests: Comic books, Manga; most things to do with Handicraft, wood, textile, metal etc, modern materials; horror, true crime; languages, art, and history to an extent
Uninterests: All things about motors; celebrities; fashion; sports; career; stock market
Feel free to PM me!