I agree with this and @TarantinoIsAMisogynist
What she said was very odd, and I don't think many people, whether they'd actually experienced a loss like yours, or even hypothetically, would ever agree with her. It's completely understandable you are upset.
But I think it is likely that from her POV she felt you were bringing something completely irrelevant into the conversation, as @MolkosTeenageAngst said, like a 'top trumps' of sadness. As you've explained, your intention was completely the opposite. But think of it if it was the other way around - in a scenario where you were getting upset talking about how much you missed your daughter, and, in a misguided attempt at cheering you up she said 'but at least you have your dh, I'm alone all the time,' or similar. You very well might have been like 'WTF, that's not relevant, why are you turning the conversation to yourself and your issues when I'm clearly upset?'
It's obvious being single is a huge worry and distress for her - maybe she's worrying about how she'll cope financially as she gets older, or she's lonely and depressed and going on holiday with her two sisters and their husbands has really brought it home. It's not the sort of thing you can always talk about. She thought she was in a place where she could confide her worries to people she trusts, who would sympathise with her, but instead (to her mind), got a 'cheer up, what have YOU got to complain about?' response. A bit like on here when posters say 'First world problem/I wish that was all I had to worry about/I imagine people in Gaza would love to have your problems.' You're not wrong - the vast majority of people would prefer to be single with their children alive - but that was not the time to say it.