Should I pass this to my daughter's ex?

Why not contact his father? If you think he would enjoy it, it would be a shame if the ex shoved it into a cupboard and forgot about it.
Probably best not to contact the ex as your daughter doesn’t seem to want you to.

If your daughter is still seeing her ex occasionally, why not simply parcel up the information and ask her to give it to him? If she still won’t give you a straight answer, you could always leave it to him and his family in your will! Putting it in a safe place in the meantime of course.

If your daughter hasn't given a definite answer one way or the other, it is up to you. Even if she had, it would still be up to you to make the decision.
If I had done all that work, and I have no bad feeling toward him, I would send it. As you say, his family would find it interesting, and both those concerned are now in settled relationships with other people. You could put in a cover note to say something like you had found this when tidying old things up, and you thought he might like to have it rather than throw it away.

just ask for the contact details of his dad and send it to him?

If it was your project at your expense then I see no reason why your daughter needs to be involved.
The man obviously meant a great deal to your family when he was engaged to your daughter and if having the family tree would mean so much to his father and siblings then go ahead, pass it on to them. I don't see the harm in doing so.

I'd pass it on.
I can't see what harm it could do to anyone, or that it undermines them in any way.

Wow! I assumed you would all say I shouldn't do it cos it could be seen as interfering. I definitely don't want to upset her. Actually, I think out of all the people who suffered during the split, it was me. He did the dirty on her and I hated him for hurting my lovely daughter. There were financial issues too, that I paid for. So he is actually the only person in the world that I hate - but she doesn't, thank goodness. I am trying to be as reasonable as she has been, in handing over all this work. I was going to pass it to his sister, but she sadly died. I just wish she'd say "yeah, do what you like - it's your work."

Wrap it up and give it for Christmas.
Take a leaf out of Cliff Richard's ghastly song.
A time for forgiving, a time for forgetting... la la la la la... somethingy something....

You will feel great once it's done.

I typed out a post and lost it!

Here goes again.
There is such a lot of work involved in researching family trees, not to mention the expense in buying certificates etc.

They've both moved on now and your daughter is happy with ^a lovely man^; you were upset and angry with him but it's perhaps time to let bygones be bygones.

I'd try to find out his father's address and send it to him. It's of no interest to you or your daughter but a great shame to destroy it or leave it to be thrown out one day.

You say he hurt your daughter badly - you can now be the better person.