This link might help ease your mind. Infant botulism through honey is kind of a "specific environment" type thing where it's not always an issue, but there might be that one time, so don't risk it type thing
Regardless of the baby probably being okay, it’s not okay that she went against your decisions as a mother. I would talk with her about this and set a boundary.
@Mollie I second this. If they dont listen to you about this and brush it off, they dont need to watch your child. Even if its a small thing, it can go worse with something harmful. Plus, as your child grows, they will push you even more if you dont stay firm. Boundaries are best set early.
That’d be the first and last time my ML babysits 😭
https://www.cdph.ca.gov/Programs/cls/idld/ibtpp/CDPH%20Document%20Library/FAQs_PDF.pdf