Whilst I feel very sorry for her I don't think you need anyone's permission to say no.
She could easily continue communicating with you by text, which would help break the ice and help you understand if an eventual meeting is a good idea?
But she wants to meet face to face and your gut reaction is that it is a bad idea. You have mutual friends, has that given you any extra insight into whether this meeting is a good idea?
Does she want closure or revenge, or a mixture of both?
I'd be sympathetic to the first aim but not the second.
What further level of atonement is she seeking? Would meeting up with you even help after a decade? Why you and not your DH? Ultimately he is surely more responsible? Could it be that he said no and she knows he'd stick to it so that you are the only moveable factor?
Only you and your DH enough knowledge to know if the original marriage and friendship was a happy one which would have continued, which case it is easier to see why the Ex is finding it so hard to get over, or if it was already in free fall and your arrival hastened the inevitable, so its guesswork to say whether this meeting would help either of you or not.
It makes me think it might be easier for exes that are co parenting to glean answers. In some cases they may have just enough contact to have the opportunity to see enough flaws in the other partner to make it preferable to be apart. How many OPs on here say that they are much happier having got rid of a cheating ex and have a better life without them. Having not had that contact for over a decade and been left wondering in a vacuum has perhaps made it harder for her.
Looking at it from the POV of the ex as an OP asking asking whether this meeting was a good idea -
It makes me think of situations where people gone NC with relatives having been badly hurt, emotionally or otherwise who might say that after a decade of NC any atonement/apology could be too little, too late.
But in this case she is not a relative, your connection was as friends. We don't know ( and I am not asking you to reveal it here) how deep or long term that relationship was, a childhood friend who was her maid of honour or a friendly work colleague. I think that would affect whether it would help the healing process or just reopen old wounds. I'm guessing that you were not close/best of/inseparable friends, but friendly enough to meet socially for a few years?
Not an expert, but could it be that this meeting is something a therapist would recommend to her? Meeting her doesn't give her back the time shes lost in a marriage... and I often see posters in the same position as the Ex advised that the best revenge is to live your best life. But i do feel very sorry for her and think that she probably needs some therapy to get over the hurt. Your DH is right that its your decision, but if you went ahead maybe he should be there too?