I doubt that a security guard would have taken it upon himself to challenge her, unless she was flaunting it. But since some fool complained, he had to do his job.
He could have just as easily asked the complaining customer why they were so offended that they felt that their sensibilities were more important than a mother feeding her child. The woman covered herself. There was nothing to see. The complainer should be told that the rest of the world isn't obligated to ensure that THEY aren't offended, and their protestations are offensive to others.
The Security Guard could have just ignored the moron's request.