I do think your 'friend' is a pretty bad Nanny but I don't think it's your job to tell the parents what you think of her. Like Outraged said, you can definitely voice your concern by stating the facts and saying something along the line of " I am worried that X doesn't pay enough attention while outside with the kids as this (state example) happened the other day, I thought you should know." It's up to them to judge if it's worth firing the Nanny or not.
But to be honest there is a lot of things that you might not know about the nanny and her agreement with the family.
For example, I am looking for a child under 1 and I am allowed to bring her everywhere I want to as long as she still has some sort of routine.
Same when I was looking after 18 months old twins so her shopping for herself or spending time at the coffee shop might not be a problem at all for the parents, but sure enough if you see her there all the time you must be there quite often as well ?
The stair thing is a no-no though as she should definitely help them and always make sure they are safe. You can't always prevent a kid from hurting themselves, but she is definitely being irresponsible here.
The not comforting kid when they are upset is always heartbreaking when you witness it but sometimes it's for a very good reason.
I ALWAYS comfort my kids when they hurt themselves but I won't comfort them every time they get upset. Kids get upset very easily and often for no reasons and most of them totally know how to get what they want by putting on a show.
My nine months old is very good at that game and she'll often start crying because she doesn't want to get changed or because I am taking too long to bring the spoon to her mouth etc... I have no reason to comfort her when she is throwing a tantrum and I won't do it as I don't want her to end up spoilt and thinking that it's okay to cry for no reason and that she'll even get a pat on the back for doing so.
I don't think that makes me a bad au pair? Just an Au Pair that want her child to be independent and have good habits when it comes to living in society.
Same with not always being all the time next to the baby. I am always in the same room and always keep an eye on her, but there is time where I take a step back and let her play on her own. It's important that she becomes independent and have time on 'her own' as well.
Keep in mind that most parents check their phone, calls, text, surf the net or watch TV while taking care of their kids and that doesn't make them unfit to look after their kids, just humans.
This Nanny obviously lake common sense and seems to have no knowledge of the basics of safety and I would definitely address this issue to the parents as you are better safe than sorry, but for the rest I would keep it to myself (I highly doubt their developmental issues, if they have any, has anything to do with the Nanny anyway.)
Good luck ! :)