I was in a very similar situation with my fiancé, not too long ago. BTC was not in question but a home was involved, and I lost big, 175k big. I will not go into a lot of detail. But you seem to have two fundamental questions.
Do you honor the gift you gave her?
Why does she what access to BTC?
I struggled with this first question, more time then I can count in my break up, I gave back a lot of valuable items to my ex when I really didn’t have to (under my own initiative). She seemed to not to even recognize that I was honoring our agreements and seemed not to care. Honestly it didn’t matter what I did, I was a horrible demon like creature that had to go. It was only later, I found out she was dating another guy. :(
But on a personal level I do see this as a question of your character. Do you honor your commitments or not? So if I was in your shoes I would not take the BTC. As I would regret stealing from someone I loved very much at the time. But I would not give her the private keys either.
Now for the second fundamental question -
I assume she wants to spend the large amount of money on frivolous things, because the money is there.
After reading your post (and coming my own jaded experience), I would not give her the private key and would let her sweat. I believe giving into what she wants is a disservice to you and her. She broke your heart and she mishandled the gift you gave her (along with not respecting your effort/time/research into cryptos). As of right now her BTC keys are not of your concern anymore, and has tough as it may seem, I think she will respect you a little more in the end for not being a push over. Besides we all know BTC is skyrocketing and she will be happy she didn’t sell now to buy a pointless gadget. And if you wait you can always revisit this issue in a year or two. Who knows things might change.
CAUTION-
I would caution you that if she thinks there is a lot of money on the line, she might suddenly become very interested in you again with the sole motivation to get access to the account.
TLDR: Her losing her private keys is not your problem. HODL
WOW!!! WOW!!! WOW!!!
This is what your reply brings to my lips. As much as i appreciate your suggestion, Something in me wants to punish her for cheating on me and having the guts to come back asking for the keys all because she now knows my addiction to cryptos back then isn't worthless. But I will think over your suggestions as it portrays the characteristics of a disciplined, principled and responsible man.