There’s always been a distinction between published work and unpublished.

If my friend showed me some unpublished work (Say 90% of the hardest work toward a significant proof) if I then go finish the remaining 10% and publish the whole thing as mine it isn’t ‘building on the shoulders of giants’ it’s plagiarism/theft.

If he’d published that work first and I took it and found another proof and credited him for his work via citation then that would be fine.