She hasn't even said not to BF - just to not do it in the church and discreetly any other time. If I had an 18 month old at a wedding I am sure that it wouldn't be an imposition to not feed during the ceremony.

Still not seeing the big deal, it is a very short time to refrain from feeding to help her keep the peace on one of the biggest days of her life. It is totally not worth stuffing up the future relationship over. There are worse things in life than someone politely asking you not to BF for half an hour.