Okay -- forgot to ask my question. How much should a 14-year-old be told. I think she deserves a soft version of the truth because she already has suspicions that are driving her nuts. And she needs to understand that H has moved on and we are not getting back together.
I would like him to say, "I have been unhappy since January, and I am seeing someone else to see if I might be happier. It doesn't change how I feel about you, or my love for you."
If he insist on dating against her wishes, I think he should say, "I know you've stated that you're not ready to see me dating right now, but this is an important part of my personal growth, and I'll make sure that it doesn't affect you."
Alas - you know you can't make him say anything smart or appropriate. I would try approaching it as emphasizing that you are not planning on dating for awhile for all the sensible reasons that have previously been discussed here, but that you can't speak for her Dad, and that he mightnot feel the ame way about it.