Something just doesn't add up here. It is a "regular" occurrence on flights and I have never, ever seen someone at AC object in any way.
I have heard an FA tell a mother she should not start breastfeeding until wheels up and to stop just before wheels down, but I suspect that was more of a safety issue (for the baby and the mother). I have heard of a woman who was walking the aisle breastfeeding and was asked to take her seat, but again, that would be a safety issue.
I find it hard to believe that an FA with any experience whatsoever would even suggest such a thing.