I would be concerned that a substrate damp enough to sprout and feed seedlings would be too damp for a healthy BP. That said, I don't own a BP, nor use coconut fiber alone, and I don't know what sprouted. My guess is, the seedlings won't last long, esp. if you aren't using supplemental light. The plants themselves are probably harmless.
An actual successful planted tank utilizes a gravel or plastic crate false bottom so excess water can collect at the bottom providing humidity and a moist area for plant roots while the top of the substrate itself stays dry.
The person your mother spoke to has a very valid concern.
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