Personally I would go & I would use a muslin wrap or walk away.
It is her day & it is something that not everyone is comfortable with.
She has made the effort to politely email you & ask you to do this for her.
Had you been there & she rudely told you to cover up then I would leave but that's not the case.
I breastfed at a wedding during the ceremony and no-one cared they even supplied a couch in the corner of the room to make it more comfortable for me.
But I was aware & willing to walk out should people not be comfortable.