I hope the woman finds someone nice to go for coffee with
I doubt this will happen. Or if she does, it'll be a one off never to happen again. People who are shameless conversation hoggers always make me wonder whether they're self aware or not. It must be a vicious cycle actually, you behave like this so people pull away, so you talk even more in a desperate attempt to keep a conversation going, people pull away even more. You end up lonely, so when you do see another person you can't stop yourself blabbering on incessantly because you know you have a limited time before that person pulls away too...
... or maybe it's just someone full of self importance who barely even notices the responses of others. If you don't notice or pay attention to the very basic need of the other person to contribute to the discussion, I doubt you're picking up on any social signals.
Either way, whether it's intentionally being self centred and rude or she's completely oblivious and doesn't realise she's doing it, being told to her face how she's making OP feel is a good thing.
The truth hurts sometimes. That doesn't mean the truth shouldn't be said. Especially when you're affecting others.
It's not like OP walked up to her out of the blue to say 'by the way, you know that you're hogging every discussion and nobody can join in with you around, right?'
The hogger approached OP! And tried to ask her for coffee, which would potentially have been more time sat frustrated while she went on and on. I'm not surprised that after the frustration of having to be subjected to it for a few weeks at the play group OP was honest. Maybe it stung, but it's honest info she can work with, and it probably stung the rest of the mums being unable to have a nice time at play ground getting to know each other for the past few weeks. I imagine if anyone overheard they'd be silently cheering!
OP, did you ever hear back from her again?