to tell this lady that he was sucking a plum stone

[Justforlaughs]

I was on the bus, and a little boy (about 2) was eating a plum. He finished it and put the stone in his mouth. He sat and sucked it for a while so I said quietly to the lady (presumably mother) that he was sucking the stone. She gave a mouthful of abuse and said that she knew that, he "likes sucking the stone when he's finished". I didn't criticise her at all, I was just afraid that if the bus went over a bus he might swallow it and choke. WIBU and interfering and would you have said anything - or even wanted to?

I wouldn't have said anything but that's because I'm a coward, you weren't unreasonable to warn her of what you thought was a risk (though if the bus had gone over a bus, you'd probably have bigger problems)
She was a little mad to have a go at you for it, a simple 'Yes, I know, thank you' would have sufficed.

I would definitely have brought it to the mother's attention, she was a complete cow to have got stroppy.

BramshawHill Whoops, yes, if the bus has gone over a bus we would definitely have had bigger problems

You weren't being rude or anything. I'd have just said "oh he always does that" or something similar.

Well, if I was the woman at least. If it was me I'd have been fishing the stone out of my child's mouth!

She sounds unreasonable to be so rude, but you don't know what other helpful advice she's received today, from other well-meaning (or perhaps less well-meaning) people which might have tipped her over the edge of reason.

I don't think YABU but from her possible perspective "I'd had nothing but criticism from friends/MIL/SIL about how I look after DC and then this woman on the bus tells me he's sucking his plum stone, like I hadn't seen him do it! Argh!"

I think you did the right thing, I would have done the same.

You certainly did the right thing. I have been present when a child was choking, albeit not on a plum stone and it was frightening, horrible and a life and death situation. A man, whom we didn't know, raced across and performed the Himler (sp) technique-all our slapping him on the back was futile. The piece of apple shot out but it has left me with a queasy sensation whenever I see a child in a situation like the one you have described or with one of those round lollies (not ice) stuffed wholly in their mouths.

The woman was rude but above all stupid in failing to realise the danger that you pointed out and no amount of 'bad day' experiences can excuse her from exposing her child to danger because that was certainly what she was doing and after you pointed it out, she was doing it in full knowledge. YANBU