Reset Iphone5 locked

I have in hands an Iphone5s, it was my GF's ex boyfriend's phone she was using for ages...
the guy doesn't want to hear about her and won't help
IEMI is clear.

I would like to use this phone but I can't reset it: an icloud account is signed in and "find my phone" is also turned on and I don't have the Apple ID password or any code.

Is there a way to reset that phone somehow? or is this phone just good for trash/spare parts?