Done that a few times, myself.
Reading this has made Anna get after me to go cycling again. She fits on one of my bikes now, and into my smallest set of bike shoes. She hasn't stopped falling over when unclipping, but she's getting better!
I could never have a bike that I attach to my feet. That would be a recipe for disaster.
Some pedals are more of an challenge than others. I use Speedplays, and unclipping is a total non-issue. I've tried others where it was a different story.
I average about one pedal tip over a year, so it is worth it to me to use them. I am not sure the type of pedals would make a difference, but I will have a look at the Speedplays. It happened so fast, that my brain had no time to react. The reason my foot was already unclipped was because I do that at every intersection I approach.
I second JJ about the type of pedal making a difference. I think an occasional fall is a given with clipless, so it may not help in that situation, but for someone having trouble learning (or fearful of trying), it's worth keeping in mind. There's also a tension adjustment (at least on some pedals) that makes a difference too.
When I first got my MTB, I was new to both clipless and riding trails and fell over every time I lost momentum or had an unplanned stop. I switched to platform pedals for self-preservation. :) When a friend finally convinced me to try clipless again, I tried the same pedals (Time ATAC) and had the same problems - unclipping took a very deliberate motion (one that I could not do at that moment when you suddenly realize that you need to put a foot down, NOW) and I generally couldn't unclip my left foot first. So then I tried his SPDs and it was like night and day. Immediate improvement, and it only took an afternoon or so for unclipping to become automatic.