I'm a "take my foot off the gas pedal as soon as I see a red light" person so that I can coast to the stop. In fact, that's the way I taught my daughters to drive.
It amuses me to see people flying past me so they have to stop at the light. "Hurry up to stop!" While I just coast and sometimes it works out that I don't have to stop at all.
I never thought of this as rude.
However, now that you mention it, LOL! there are times that I'm behind a coaster when I want to make a left turn and miss the green turn arrow, because I can't get to the left turn lane. I honestly do think to myself "You know . . . if you hurried up to stop I would have made that light!"
But in the grand scheme of things, it doesn't ruin my day.
Nor do I think it's rude.
She thinks it is extremely rude to slow down so that you don't have to stop while the light is red as it means someone behind them may miss the light if they are going to be turning.
The people in front of her can't read her mind.
Maybe she can put on her turn signal early and hopefully the person in front of her might get the hint?
I tend to look in my rear-view mirror when I'm in the right hand lane and not turning right. If the person behind me has a signal on, I'll try to move to the left lane if possible.