Will a lawn fertilizer (30-0-3) cause flowers to NOT bloom?

Her concern is reasonable. Excess nitrogen is not good for flowering perennials.

Depends on how much gets in there. A little bit probably won't hurt, but if you're talking a standard application, well then you'll probably end up with much taller plants and potentially fewer flowers.

I made that mistake my first year of gardening, when I was a teenager. I later used it as an example of over-use of fertilizer in my talks I gave as a master gardener: "The first year I grew broccoli, it was four feet tall. The stems were thick as baseball bats. I was so proud! I waited all season, but it never set heads..."