Good grief.
She clearly said that his ex was abusive to the point the poor guy has mental issues trying to deal with it. Why is it so hard for certain people here to read things as they are and not paint over everything with a gloss of 'all mums must be perfect'. Guess what, some aren't. Some are abusive, and when they are the dads have little to no support, and close to zero chance of getting their kids away with them. This is exactly why - women like yourselves refusing to accept the possibility of mum the abuser. Well it happens I'm afraid.
Its irrelevant how soon OP got together with a single man. There is zero morally wrong with that. Again, were the sexes reversed how you would all applaud a man 'taking on' an abused woman and her kids. Sexist much?
There are 2 people responsible for the breakup of any relationship, guess who they are, the people in it. No matter how head turning a person is, they're not responsible for your marriage. She acted extremely decently, clearly it sounds like the opportunity and feelings were present to cheat, and they chose not to but to put clear space between relationship a and relationship b. It's very easy also to be abusive to someone and then get affronted when they have the audacity to leave, 'how dare you defy my control'. Leaving abuse hardly makes you the one who 'broke up a relationship' actually, arguably the fault could lie with the abuser for making things untenable for the partner.
Imagine just for a second that what the poster wrote was accurate, and her partner was nastily abused, exactly how spiteful and unhelpful are some of the things written here. How about read things how they are written instead of trying to find any possible argument in which in fact mum must be faultless, and accept responsibility where it lies, not with a step parent. It's not actually our responsibilities to fix your kids after you fail to keep your relationships together, or our responsibilities you couldn't do so. If we choose to, how about a thanks.
Apologies for the rant January, I'm sorry for what your man went through. It must be very difficult and confusing for your dsd, and what I do agree with that was posted above is it's all about time, be yourself and you really can't rush things. I have across the years been asked all sorts of things, found out that mum said all sorts of hateful things, and most recently that dsd had confusing feelings about marriage coming into things. If dsd sees you being consistent, and not being the bad guy that she worries you might be, over time she will accept that's the case and feel confident that she knows what to expect from you.
I have openly said very honestly to DSD as she grew that I am sorry that her mum and dad aren't together, agreed with her that it's actually not fair, and told her that actually there aren't any such things in real life as goodies and baddies. Real people just have different perspectives and sometimes grown ups don't love each other like that any more. I think around the age of 9 she saw another friend's mum and dad break up in a much more healthy way, and they used similar language to this, so she started to see and accept that in her parents. Unfortunately it's a very grown up concept and it takes a long time to accept, and perhaps in some part they never will and will always feel a bit sad mum and dad didn't raise them together, and I think that's ok, it is sad.
Of course the one thing you must never, ever do, no matter what, is say anything negative about mum. You sound like a sensible person who has figured this one, but no matter what, smile sweetly, behave well, and accept that it's better to just stand accused of being the bad guy than to tell a child their parent is abusive. I think sadly they see in time if they aren't nice people, but that's still their parent and it's the one way you will completely alienate a child. So smile and talk about perspective, and be yourself. She sounds like a really nice child.