He's asked me to have a word with some people at work who have been giving her a hard time because she is finding it upsetting
Me, at your stage, would have been having a word, asking them to step it up a bit! Not that I am recommending that.
My ExH had an affair and left me for the OW when DD was 9. That was horrible and I was not entirely reasonable through all of it.
I did insist that we get together before DD was allowed to meet her. Ex insisted on a phone call to OW first. She asked me why I had to meet her and I replied that if she was a mother she would understand! She then tried to gain some sympathy from me by saying 'That's where you're lucky and I'm not, C does not want any more children, so I will never have any'.
I told her she could not expect sympathy from me, she could have chosen a single man without a family, but as she had chosen a married man with a family, she had only herself to blame.
I never got a minutes trouble from her after that. We did meet up, it was civil. She married him recently, but they 'kept it secret from me, because I would go psycho'. In fact I met someone a year after he left and we have two children together and have now been together longer than me and him were. Oh, and his eldest daughter, who had been my step daughter had told me about the wedding weeks before.
I managed to continue to not give a shit.
You will get to this stage one day!! I promise.
For now, do not message her, that gives her more power than she deserves!